1997 Chevy Monte Carlo

I went out to start my car this morning and it started up fine. I turned the defrosters on and after a few minutes, I noticed that there wasn't any warm air coming out. I didn't think anything of it because the heat is usually slow but it's never been this bad. While I was driving, the thermostat in my car kept fluctuating between hot and cold. The heat seems to work fine when I'm accelerating but when I have to slow down, it starts blowing out cold air and the thermostat reflects it by getting warmer. But then when I accelerate again, the heat works and the thermostat goes back down to normal. I'm not much of a car guy and I was wondering what I could do about getting it repaired?