Where should I start?
Pick a module and start changing things. First thing you need to do as a new module maker is learn the engine. Last I checked, Zombie Horror was generally the most friendly to new module makers. I'd recommend you download that and start making changes. Like Shoob says...Dragonball T is monstrously large. It may be more than you'll want to tackle to begin with.
I want to make the skill system classless kind of like the Dragonball-T module
DBT's skill system should be easy to port. The relevant script is scripts/enroll.lua. I don't recommend you start adapting it as the first thing you do, but I think it's safe to say that once you're generally comfortable around the engine, adapting DBT's enrollment system won't be terribly difficult.
I was thinking I should try to just implement a basic arena itself first, then add in some extra random dungeons to loot, and add more depth to the character growth and arena later on
Yes, that would probably an excellent way to start out. Why not give it a go, and ask here once you start running into more specific things you'd like to know about? T-engine doesn't have a lot of documentation, so jumping in really is the best way to learn.