Ok I bought a new house and went through this for my backyard. Front yard was sod but back yard was just random patches of grass seed thrown around. So the big question is how much are you ready to spend? A few thousand dollars and careful watering can replace your whole lawn in a hurry but since you posted here I am assuming you are looking for the cheaper method?
Scotts or some other lawn care service isn't a bad start for $35-70 every X months but you will need to be proactive.
1. First thing you'll need to put down is some kind of
weed preventer. Spread it liberally.
2. Peat moss and grass seed. Go ahead and buy a big bulk bag of grass seed ($100 for 50lb bag I think) and spread that out and water it carefully but regularly. A well seeded lawn will prevent weeds from growing more then anything else you put down.
3. In the fall put down some form of grass fertilizer and another round of weed preventer.
4. Hand pull weeds. use tools or try your hand at painting the weeds leaves with grass/weed killer carefully but this is a tedious process no matter how you do it.
5. Milky spore - Kills those grass root eating grubs
Next year you should see a noticeable improvement though you will likely spend the next few years working to fill in every spot.
This is what I did after I let my lawn go to weeds during my first year. Now it's green but there was a spurt a year ago where grubs started eating patches but that's nearly fixed now to. As to what specific products you should buy it really depends on what weeds are growing in your yard.