I have a gap in my front teeth how do I get rid of it?

So when I was younger I had a really bad gap but as i grew older it closed. I noticed one day that my gap is reappearing. I have straight teeth the problem is the gap and it's not that noticeable but it bothers me. Do i need braces or retainers? How much do they cost? I'm a freshmen in high school so I don't want braces for my high school years.