Can i cook chicken that went off yesterday? It smells fine?

When in doubt, throw it out.

Sell by date does not mean expired. Expiration date hardly is ever a true expiration (because stores want to sell it before there's even a chance that they're held liable for selling expired food). It's good to go as long as it doesn't smell funny. 

What do you mean, "went off"??

Yes, you can as long as it isn't slimy or smelly you're fine. Thr sell by date is usually 2 days after I buy, in our store. I'll only keep in fridge a day or two, but then freeze it.

Of course.Cooking is the best method for killing any harmful bacteria.