So I do own PoE on steam...I've tried getting in to it several times. Is the Marauder a decent class to start with? Also, what's the most "popular" mode to play?
Classes don't really matter much. They define what you look like(probably the most important aspect in fact), where you start in the tree(relevant early game but rarely late even though some classes have better affinity with certain specs) and which skill gems you're offered as quest rewards while you level(relevant for the first char on a new league kinda, but really it takes a few seconds to trade for gems you want generally). Every class can be anything though, you can make a tank melee witch and a caster marauder. The starting point in the tree will mold your choices a bit however, for example a marauder caster would have to get several life nodes then several empty nodes to get to the damage nodes and the fact you don't start close to damage nodes might reduce efficiency of the build, but we're talking min maxing high end stuff pretty much. Also in that particular example, marauder casters are in fact very good eventually but are a pain in the ass to level due to shit int.
So kinda pick one you like how it looks then make a build that uses the early tree strength of that class, or alternatively do it the other way, but you can't really go wrong with a class choice.
As for the "mode", they're leagues. There's the typical normal and hardcore permanent leagues, then they always run like 4months leagues with new rules. Those are the popular ones. Once the league is over, your character is transferred to the permanent league and a new one starts, think D2 ladders. Difference being, each league tends to have specific rules added to them and they often run race events on short term leagues(or not so short), like 2hours races and shit but also sometimes 1month races. On hardcore in this game, first it is fairly hard and second your character isn't deleted unlike diablo hardcore. When you die, your char is simply moved to the permanent standard league. You can still play it and stuff but not on the HC league you were in. Your stash stays there too for other HC chars, if you want to change you just make a shit mule, load it up and die, but you can never go back the other way.
They're starting new 4months leagues next week so might as well wait for that. The new leagues will be Rampage(the standard league, the added rule is that you get a bonus for killstreaks) and Beyond(the hardcore league, they haven't released the details of that league yet). You can mess around with current leagues if you want, I'd recommend Ambush/Invasion for population since they're the current 4months leagues, unless they're over yet not sure. Standard/Hardcore is fine too but they tend to be deserted during 4months leagues.