Not sure how to approach heater problems
 

I have a 1991 Toyota Supra And my heater is acting up. It will begin to heat up like normal as the water normally does but then right before it gets comfortably hot it begins to blow cold air. And then about 5-10 minutes later it will go back to hot. I'm not good with cars whatsoever and I'm not too sure how to even approach this. Any help would great!