i have a B1/B2 Visa and now, i'm in the USA and applied to study in the university, i'll get I20 and want to get F1 visa, can you help, costs & time?

What you need to do is apply for a change of status from a visitor visa to a student visa. The time would depend on the Office that would adjudicate the petition but it can be done. Keep in mind you would have to explain that your original intent was to visit and then after you arrived in the states you then arrived at the idea that you wanted to study here. Keep in mind that if you do succeed in getting the status change that if you ever exit the country and then return you must reapply for a student visa as the i94 card must match the visa stamp in your passport. Your other option and perhaps the best is to go home and process at the consulate there for an f1 visa as it is faster and u won’t be out of status here if you apply here and it is denied.  Counsel anywhere in the USA can assist you.