"decks" stuff looks incredibly confusing.
The game is a lot simpler than it first seems. Basically you just get a skill wheel which is split in to categories. First it's split in to weapons so you have a Blade section, Assault Rifle, Elementalism section and so on. And then each of those weapon sections are split in two, one usually being more protective and one being more offensive (but it doesn't always work like that).
You can use two weapons, so pick one and use that with anything else. Even the Assault Rifle only uses one hand, so you can have that and then a blade or blood magic or whatever else you want.
How you should play is to decide on a role you want to fulfil. It's just like other games, you can tank, heal, dps, or support, or you can try to mix things up a bit. But generally groups are dumb will only want one of those main roles so you might find it better if you can be uber at just one thing. But anyway, once you know what you want to do, just look at all the skills and work out what would be useful to you. It's a lot of skills to read which is hard at first, but pretty soon you will get a good rough idea for what is where (and there is a search tool built into the game too). For example only a few of the weapons can do healing, and only some of the individual categories within those weapons. So for example Elementalism is only really about dps/support so you can't be a healer with that. But blood magic, blades, assault rifles, etc.. have some nice healing abilities.
The early part of the game is quite simple because you just pick skills that you want, and work towards unlocking them, and then that's it... A heal is a heal and you are now a healer. Make it stronger by getting gear to boost that, and you'll go far. It only gets more complicated at a higher 'levels' when mobs are harder and things get more serious. At that point you will want to start experimenting with abilities that complement each other. So for example you might find a passive ability (or even an active one) that provides an extra heal boost when you attack a mob that is 'Afflicted' with something. So if you then find an ability that causes Afflicted when you attack (or a Passive that makes you cause afflicted), then you can Afflict stuff yourself, then hit it with the other thing and cause an extra boost to your heals. This also works for damage and tanking and whatever else. There are 'synergies' all over the place, and it just becomes a game that's all about exploring these and working out powerful synergies that give you a big boost.
The combat itself is not all that interesting, you just spam your half dozen buttons without all that much thought. But planning out what those buttons should be on your hotbar, is quite deep.
