Well I'm not from a place that gets terribly hot, but the summers up here can get pretty warm.
I have a buddy who carries a full size XD 40 during the summer with just a loose fitting un-tucked t-shirt.
The t-shirts that I wear are not so loose fitting so I resign myself to pocket carry with an SP101 (it is somewhat weighty but it works for me) during months I can't wear a button down shirt.
All forms of belt carry require me not to tuck in my shirt, which I usually don't do anyway.
If you want to belt carry, I highly suggest inside the waist band. At first it sounds uncomfortable, but with a semi auto (flater then a revolver) it works great for me. A good quality holster is important.
Suggestions for a carry weapon will be very varied, at the moment I am using a S&W M&P45 compact. Suggestions for caliber will be just as varied, but it would help to know what level of experience you are at.