There is more than one way to fix up your lawn. If you can afford to replace it with new sod, that is one way. The old dirt needs to be removed first. Expensive and labor intensive.
Some things that benefit existing lawns are thatching, aerating, compost, reseeding. I'm not a lawn expert, however. Lawns look great at first and require high maintenance to keep them looking nice. One a lawn is neglected, it is hard to get it back to looking good. You might want to try the library. They probably have some good books on lawn care. I don't care for lawns myself because they require so much maintenance to keep them looking nice. I'm replacing mine with lawn alternatives and gardens.