I pile on more wax every session. I can really tell when I don't too, because I start to slip all over the place and fall. I only take all the wax off when it's starting to get really grimy every few months +. I'd take your bar and put it on top of whats there unless you really feel like cleaning it off and starting fresh. Rub it around in circles by where your feet will be and on the rails so you can hang on to the board. Cover a large area because if you're learning you'll probably be all over the place when you stand up.
I wear a rashguard or a wetsuit every time I go out just because of comfort to avoid.. rashes. It can be painful if you're out for awhile, at least for me. Wear it for the sun protection and rash protection if you feel you need to. You'll probably look funny either way if you're just learning.
I can add more to this or answer whatever questions but I have to run to work so peaceeeeeee