Which class you pick first only influences which city you start in and your questline for the first 15 or so levels. Once the story sends you to the other cities, everything syncs up and the story is the same for everyone going forward. Playing as a tank(marauder in your case) typically gets you into dungeons faster, but if you arent comfortable doing it then sticking to ninja or dragoon will work out fine. And the game doesnt punish you for switching(it actually rewards you with bonus exp to your lower level classes), so feel free to experiment.
As for the chat issue, if you havent already try to get an invite to your servers Novice Network(sitting in limsa/any city near the market board and shouting for an invite will probly work). Its an invitation chat channel where newer players can ask questions of more experienced players. Every servers NN is a little different, so i couldnt tell you how chatty Zodiark is, but at a minimum it can get you some help if you need it(and maybe hook you up with a good FC too). Failing that, you can use the Party Finder tool to try and find a good FC. One night when you have some free time put up a PF group and explain what kinda FC you want. Odds are youll get some info from a few recruiters and most likely an invite.
Finally, when it comes to tanking advice... ill be general and leave the good stuff to the pros. In general, you want to grab the attention of all the enemies in a group(at higher levels this can be more then one group). Use your AoE move(s) on them(like overpower). Then run past them and turn around. This will get the enemies to look away from the group and at you. You want to use your defensive skills(like rampart) to take less damage and make things easier for the healer. If you have any good self healing skills, these can be mixed in if you feel like youre in danger(but in general your healer should be able to keep you up). Once the enemies are grouped up you can use your single target attacks(still mix in AoEs too when you have good ones), and just keep switching targets every hit or two so that everyone stays focused on you. If you lose control of an enemy, try to get it back(something like provoke + tomahawk). Ideally you want the enemies to stand close together so that your party can use their AoE attacks to hit everyone at once. And once you have a hate established try not to run around too much - some of your DPS friends need to stand in certain positions to make their attacks work and if the enemies wont stay put they cant do it. For bosses this can mean dodging an attack during the wind-up(they typically cant move while charging a big attack), then moving back to the same spot once its safe. Beyond that, try to learn which attacks can/should be stunned. If you can manage that much, youre already better then probly 1/3-1/2 of the tanks out there. I would also say dont be afraid to say that youre new and dont be afraid to mess up. We all had to learn sometime.
And of course, any more questions feel free to ask. Best of luck, and welcome to Eorzea!