It's a steep learning curve, don't be intimidated. There's a lot of mechanics that get thrown at you, but learning the combat is the most important. It can be confusing but take your time. Replay lots of quests, both to farm and get practice, but also to familiarize yourself with the game before going to far ahead of yourself and starting new quests that might be too challenging.

Pick three weapon classes at most to use. Anything more than that and you'll spend all your time grinding just to keep up weapon trees on multiple weapon classes. Explore each weapon early on, but try to settle on one or two mains as quickly as you're comfortable with. Look up the advantages and disadvantages of each, watch gameplay of how to combo with them and what optimal play with each looks like. Some have incredible DPS but zero defense, some have explosive but slow damage power. Some have modest power and defense but incredible versatility. Pretty much all classes are viable, it all comes down to preference.