I'm starting a final clean up business we clean homes and apartment complexes afetr the construction is done mop,vacume and windows,my question is how do I bid jobs how much per square foot

Something competitive to your area... that you can still make money. You won't be the only place bidding... and they have the experience to know what it really costs and allow enough to make a profit without overbidding and losing the bid, or underbidding and losing money.

You get access to other bids and then you bid. These jobs are posted somewhere. On line?

You should know, from experience doing the work, what you have to charge to make money.
Also you will find some new people start, with no idea how to do this, or what is nvolved and underbid very badly.

You need to meet and sell your service to the people. Lowest bid does not always win, in a good market, experience good relationship means a lot.

And to be honest kick backs to some management happens in some places.

Selling your service is as important, if not more important than the bid.