You should be able to build a portfolio in time for college applications, however there's no 'general' portfolio you can make that can apply to any college. Do you already have pieces that you're having trouble figuring out which are portfolio worthy or do you want to make new pieces?
A lot of them have specifications as to what they want to see in the portfolio, A college like Calarts are looking for different things in a portfolio than say, RISD. Mine wanted to see at least 25 drawings from life, 5 personal drawings, and so on. This is so the college knows what potential you have and whether or not you'd be a great fit for the program. That is to say, there are colleges that don't require a portfolio upon applying. Some only require it if you want to apply for scholarships.
Just figure out what colleges you want to try to apply to, look at the accepted portfolios, and if it's nearby - you you can ask to set up an appointment with a counselor so they can help you prepare a portfolio that can give you a great chance into getting into the school. There's also things like National Portfolio Day that you can bring your portfolio to representatives of the colleges themselves and they can tell you what you're missing, what to work on, and so on.