Lawn problem, novice here with lawn care, please help?

i have a big backyard, and i am from upstate NY. i want to have nice green level lawn, i don't know what to do. I went to Lowes and the gardening sales person sold me soil, maneuer (i don't know if it spelled like this) and something to rake and aire the lawn. the lawn looks bad, it has leaves and the gr that grows in some parts is green in others is brown, etc. I don't know if i should just get someone to come and do the whole yard for me and level it, or do you think i can do this myself one spot at a time. Please any help any ideas will be appreciated. Do you know it it's expensive to get this done? Thank you in advance, please let me know.