Hmm.
Sometimes people will help you and sometimes not. Normally the nice people will help you. There are normally two types of party leaders. There's 1) The one who is in full control of the party and they listen to what he says and 2) A more democratic one, that essentially is just agreeing to create the party because no one else will. Either one can be fine.
To making a party:
Tank: Pld or nin. War/nin can tank, but I always hear a lot of them don't want to. I'd ask the war/nin if he can tank first.
DD: You have to make sure these two can make a good sc against the mob you're fighting. As a thf, I know who I can and cannot sc with. Early in the game I can sc with rng, drk (w/ great sword), drg and sam. I can only make distortion again with drk through Vorpal Scythe >> Dancing Edge, and a lot of drk's hate that ws, so they won't like the party. I can only make distortion with a mnk at 60+. I can only make a light sc from 66-71 with drg, rng, or war (also the only time a war can open a sc for thf that uses thf's best ws) And so on, you don't need all the info. But have a sc chart handy so you can figure out a good one.
Healer: A whm is pretty much needed 60+ if you have a pld or war tank. If it's a nin you might not even need a healer, just someone who can heal in emergencies. Normally you should always have someone to haste so whm, rdm or smn should be in your mage lineup.
Refresher: Rdm or Brd. Many people have decided which they like most by 60-65+. But most people also agree both are better, heh. With a pld tank it's normally easier to get a rdm, as a brd needs to get the pld to move a certain way, or sing an extra song. There are some awesome brd's that can get by this very well, but not all will be so.
Mage Damage: You pretty much should have a DD in the mage section. Unless you have like 2 rng's in melee you don't have enough damage. Rdm, blm and smn can fill these positions. A smn later in the game is awesome damage, not quite sure how great earlier. But they can MB for 500+ and use Fenrir's Eclipse Bite for 500+ as well. Blm is the *best* nuker, but they're not always around. Rdm can also refresh or heal if needed, and a smn has some nice party buffs (blink, stoneskin, acc/eva boost, all stat boosts, etc.)
And if you couldn't tell, normally a party is 3 melee/3 mage.
Now, let it be noted that there are other odd combinations of parties that will work, and will work FINE. This is just a guide to making a solid party. Unless something goes wrong I can always make a party that makes 3.5k-4k exp/hour going with this formula. But it will take time for you to realize the pros and cons of jobs, and you have to know that not all jobs are perfect. Even rng's or blm's. Both can steal too much hate, some rng's miss ws a LOT (even with sharpshot) and that affects damage, some blm's nuke too hard and don't debuff. Drg's are an awesome melee class. They hit for a good amount and has the best natural accuracy out of all the melee classes. They're a very good sc opener and an awesome trick ho.
I hope this helped you out, if you have any more questions I can help to answer them, just ask away :)