catas is the only place you can level up, other than skilling but dont worry about that quite yet. its way easier to level up if you have other people with you, but it'll be hard to find people when you just start out.
you get the next class once you get to level 71 and attempt & pass the master quest
power strike is very good for playing alone, vitality is also going to be good. but balance it with str/dex depending on your weapon
LS isnt and easy game these days, going to take a lot of time and effort to get good