Hey,
I'm still new myself but it's fairly simple, go to: https://heatwallet.com/#/login and create a new account. once your account is setup you will see icons in the top left, select the option "exchange" and from there you can load bitcoins by using the load BTC button in the middle of the screen. Essentially it gives you the address you need to use to load bitcoins, so you take the address listed and go to wherever your bitcoin are and send to the address. My transfer took about 5 minutes to go through, once you have bitcoins in your heat wallet you can buy or sell using the buy/sell option.
Apologies if it's not the clearest because as I mentioned I'm fairly new to HEAT myself, I must say that their tech is the most interesting I have seen in quite some time, they have a lot of potential.