Steps to a better lawn:

1. Soil test. You can't have a great lawn with poor soil. It will save you money in the long run as it will eliminate unnecessary fertilizers/amendments. Logan labs, $20 basic soil test, www.loganlabs.com.
2. Kill weeds with a liquid weed killer(s). Weed and feed products are an ineffective means to kill weeds, and it provides fertilizer at the wrong time. Spray weeds with a product like Weed B Gone Max via a hose end sprayer or tank sprayer. Some weeds require Weed B Gone Clover, Chickweed and Oxalis killer, and some require two doses two weeks apart.
3. Apply a preemergent herbicide like Dimension, Stonewall, or Barricade now to kill all future germinating weed seeds, especially crabgrass which germinates when soil temps get above 60 degrees. This will also prevent new grass seed from developing, which is fine since you should seed in late summer/early fall. The preemergent should have worn off by then, but read to label to make sure it wears off about a month before you do any seeding.
4. Raise your soil's level of organic matter via regular applications of organic fertilizers like Milorganite, corn and soybean meal, compost, and mulch mowing. Your soil test will indicate the percentage of organic matter when you get it back.
5. Mow your lawn regularly at the correct height. Most cool season grasses do well around 3 inches. Some prefer a bit longer, some a bit shorter. When mowing don't cut more than one third of the blade at a time. So if you want to keep your lawn at 3 inches, then mow it when it's 4-4.25 inches tall. If you like 2.5, then mow it when it's about 3.25 inches tall. Taller grass needs less water, and keeps the ground a bit cooler, but too tall can lead to problems.
6. Water your grass when it needs it, and do it all in one shot. Short daily waterings encourage shallow rooting. Most grasses need about one inch of water per week but this can vary. Some areas might need more, some less depending on proximity to the street, shade, soil, etc. Do your watering in morning before sunrise to reduce evaporation and disease.
7. If you do use synthetic fertilizers do so at the correct time and the correct rate. For most cool season grasses this is in mid to late spring (May), early September, Late October, and late November (this one should be quick release nitrogen applied when the grass has stopped growing but the ground isn't frozen). If you use all organic fertilizer you can pretty much apply it whenever. Never apply synthetic fertilizers in the summer. Organics can be applied in the summer, but I would cut down on the rate and frequency a bit.
8. Assess your lawn in late July/early August to determine if you want to over seed, or renovate (kill all grass and start new lawn). If you don't need to seed then apply preemergent again in late August to kill winter annual weeds.

On tiemco's step 6, frequent watering is what weed seeds need to sprout. If you water like he suggests, you should have a lot fewer weeds.

Fescue lawns tend to thin out for various reasons. Once they thin out the only way to make them dense again is to add more seed. Many people will mix fescue with Kentucky bluegrass so the bluegrass will spread and fill in when the fescue thins out. Thin lawns allow the weeds in to take root.

Since you have a weed problem now, I would raise the mower all the way to the top until you notice the weeds are declining. Tall grass shades the soil cutting down on the weed seed germination.

they use professional weed killer products like RoundUp pro Max or Rodeo Herbicide

NightSkies, the professionals go through as many as three years as a 'cardholder' under the supervision of a state certified "Pest Control Officer" in a lawn care business.
(S)he then takes an written examination to determine that (S)he knows enough about the laws, effect on the environment, label limitations and similar topics to be let loose on the unsuspecting public, that is, (S)he can operate a chemical application business. (S)he then has to obtain enough insurance protection against a litigious public, invest in sophisticated application equipment and compete against "fly by night" unscrupulous operators to eke out an existence.
That said, you the interested and engaged homeowner, can do much to obtain that pleasing patch of grass between the street and your castle.
As an aside, that patch of grass is almost embedded in your DNA because - directly or indirectly - Poaceae or Graminaceae is the family of plants that feeds the people on planet earth. (Except those whose primary article of diet is the potato).
Here is what you can do.

1. Grow a healthy lawn.
   I know, I know, easy for me to say. Patience!
   Treat your lawn like you treat your hair - mow it, rake it and shampoo it, on a regular schedule.
2. Mow it at the correct height for your particular turf ... with a sharp blade ... that has been previously cleaned (fungus prevention).
3. Mow often enough to prevent weeds from seeding.
   So now you have to know the life cycle of the weeds. It is very easy in this era of the internet. You will have to learn the scientific name of plants if you intend to do this scientifically (pardon the pun).
   Your neighbors will laugh when they see you mowing 'thin air' but you can afford to be smug knowing that you are preventing weeds from (re)seeding.
4. Scout your lawn on a regular basis (twice a month) to determine whether insects are present. The professionals go by a 2x2x2 formula. 2gallons of water; 2 tablespoons of lemon flavored dishwashing liquid - poured over 2 square feet of the lawn - gently! You want the solution to go downward and flush out every living thing for you to identify ... and determine whether it is harmful or harmless ... and whether it is above the threshold for control. So now you have to know the insects, their life cycle and what to use to control them; if indeed control is necessary.
   Look, I have to leave this now. We can resume, if you like.
   In the meantime, check out This site.

Growing healthy turf is always the first step in the fight against weeds. Pre emergent use in the spring and late summer to prevent summer and winter annual weeds is a big help. Really the secret is not the products that are used, though that is part of it, timing is probably the key. Annual weeds you need to spray when they are young plants and by the time most annuals get to seed head stage they are impossible to kill. Perennial weeds like clover, dandelion, ground ivy, and bermudagrass persistence is the key spray these weeds late summer through fall. I am not familiar with the laws governing pesticide use in Illinois but in my area a home owner can apply any general use pesticide on their property. General use pesticides include: most 3 way products, including Trimec, for broadleaf weeds, also Drive and Acclaim for grassy weeds in your lawn. Of course there are a lot more options out there but these products will take care of the large majority of weeds in cool season grass.

NightSkies, Trimec⢠is a very effective plant killer. It is a combination of products, one of which is 2.4D.
2.4D is a hormone that causes plant cells to grow more quickly than they can obtain nourishment. Typical symptoms are grotesque growth and multiple budding.
2.4D is deadly to St. Augustine. for example.
On the other hand, atrazine is a product that is very effective against plants that grow from seeds; not so much against plants that grow from cuttings.
Do you now see why - in Florida - atrazine is an effective weed control chemical on St. Augustine?
And 2.4D is recommended for Bahia lawns?
Always remember that a weed killer is first of all - a plant killer!
And the the danger is in the dosage.
If you see St. Augustine listed on the label of a product containing 2.4D, please observe all the directions to the letter.