1998 Chevy S-10 several problems

While I was living in Texas, my truck was overheating. I got it to a gas station and put some coolant in it and it ran just fine. Before that, my truck would not put out any warm air in the defrosters. I'm back in Illinois, and it's 3 months later, still no heat coming in through my defrosters, however, when I put antifreeze in my radiator and my antifreeze reservoir, I get heat. Eventually, it's not even doing that, and I discovered that the cap on my radiator is not coming off as easily as it should. After filling it up several times with antifreeze and seeing no leaks, it eventually starts becoming noticable there is a leak. It almost over heated on me going to work, and I put some antifreeze in it to tide it over just so that I could make it home. I believe the source of the leakage is the water pump because I see smalls drops of coolant coming from where the belt is.

Questions here: Why is my truck not bringing in any heat? Why is it over heating? Do I need to replace my cap? Could I very possibly have a bad head gasket? How do I fix these problems?