The 10 Commandments of Monster Hunter
1. You will get better. -- If you find yourself saying "Fuck this game is so hard!" Do not worry, you will get better, everyone does. The best way to get into the game is to play Multi and Single at the same time. No one starts out awesome.
2. Choose your weapon based on your playstyle not on power. -- I know everyone sees the power of a hammer or a great sword and thinks "More power = faster kill!" NO! In the world of Monster Hunter 100% damage is almost never guaranteed. The higher the power the harder it is to swing and the harder it is to get the damage you see in that little weapon information window. That hammer may look badass in your hand, but you won't feel like a badass when you get tossed around by a weakass punk like Dos Jaggi.
3. BE PREPARED! -- You should always have 10 great potions (once you can make them) 10 potions, 10 meat (once you can start making your own, start making them) one trap of each type, 2 trap tools, 5 or so of the items necessary to combine with trap tools to make extra traps and a few paintballs for marking your prey. You can carry 4 traps at a time, so when you can do so, do it. People will love you for it. Sometimes they have sandstorm sales online that put a lot of this stuff on the cheap, so take advantage. You can get honey which is a key ingredient to making Great Potions in the single player farm. When I said play single player and multi at the same time this is what I mean, your gathering in the single player will make your multiplayer experience go a lot smoother. Oh, and nuts are great. Get a lot of nuts.
4. Learn the monsters. -- When you know the monsters, you can abuse the monsters. If you know their escape patterns, you can set traps way in advance to punish them. If you know their attack patterns, you can know when and how to attack them. If you know their flight patterns, you can minimize your kill time because they won't be able to fall asleep when they are low on health. If you know their weaknesses and their tells you can paralyze them and turn that roaring dragon into a drunk idiot. The game at it's heart is punch-out. Don't forget that!
5. Be nice, help the new guys. -- Everyone starts at the bottom and getting the hang of the game is hard to do alone. If you've already gone through the 1 star quests, don't think you're too cool to help someone beat their first Qurupeco or Dosjaggi. You can always get materials and stuff.
6. Unless you have your math worked out, buy armor in sets. -- Each piece of armor you get gives you a point or two in a certain armor skill, these points are pretty much useless until you have 10 15 or 20 points in that one skill, the same goes for negative points so don't be scared to wear something that puts you in the negative. Armor skills are the life blood of customizing your character. You can get things like auto monster tracking so you never have to use a paintball again, elemental protections, and all sorts of really, really useful things. Just remember, sets.
7. Know your weapon trees! -- The weapons you can make in Monster Hunter generally come from the weapons you already have. There are only a few base weapons that you can make at any time. Those weapons, through upgrading, then branch and unlock various other weapons. Typically you'll want to have 2 or 3 weapons you're upgrading at a time. The more weapons you have the more options you have, never rely on raw power alone, status effects like sleep and paralyze will help your party greatly but some monsters have their resistances and you want to be ready for that!
8. Communicate. -- If you know where a monster is, let everyone know. If someone helps you out, thank them. You are a group fighting something much stronger than you, if you don't help each other out and communicate, you're bound to fail. If you want to go out and mine  or gather, TELL PEOPLE.
9. Don't jump straight into guns. -- Guns require a separate set of skills and a separate set of armor. Learn the monsters and the game before you attempt getting into a range weapon. Gunners are very helpful because they can heal and do all sorts of things, they are the caster class of the game after all, but just know that it's not an easy road.
10. Eat! -- Online (and eventually in single player), before every quest always go to the dinner table and order some grub. The bonuses you get from food will make your life a lot easier.
That's basically it. Just remember to have fun and if you fail, you fail. Treat your partners with respect and work together to get whatever it is you need to make the next step up the MH ladder.