How to turn off check engine light

Hi,
I have a Mercury 2000 cougar with v6 it has 114,000 miles on it. Awhile back I replaced the battery then it drained that so we replaced the alternator. Now I pray that works. But how do we get the check engine light off? Is there a way with out taking it to the garage? I hope the new alternator is okay and that was the only problem. But we don't know until the light goes off. I just don't have much money and I need to get that light off. Thanks Susie