(These notes are for NORMAL, not HARDCORE mode. Not yet. And on Normal difficulty.)
Okay -- you've gotten your character up to level 4 or so, you've picked your first skills, and you've even got some gear. You're on your way to defeating the Big Bad Boss at the end of the game -- and unlocking the increasingly harder Nightmare, Hell, and Inferno difficulties.
Now -- let me add a note up front, here -- this is YOUR game. I have some tactics that I use, but this should not be considered "set in stone." I use "whirlwind" a lot, but you might prefer to have Seismic Slam or Hammer of the Ancients (both of which cost less energy) on your right mouse button. I prefer to play with a shield but you might like the two-handed weapon best.
It's your game. Set the game up however you like it. Experiment with things -- that's what makes games fun! If you have a group that you play with, then talk to them and ask them how they'd like you geared (the strategy may be for you to run in and take lots of damage while everyone stands at the edges and picks the little guys off and someone heals you, for instance.) But if you're playing it solo, like I do, just make characters and have fun!
At "Normal" difficulty, the game is a lot of fun. You can kill almost everything, and other than the main bosses, making it just hard enough to be a challenge but not Disgustingly Hard.
For the entire first game (Normal mode) my bar was set up with Ground Stomp (1), Revenge (which heals you) on 2, Weapon Throw on 3, Call of the Ancients (you get allies for a few seconds) on 4, Cleave on left mouse button and whirlwind on right mouse button. This worked well for all the bosses -- Call of the Ancients, which summons three legendary heroes to your side (for about a minute) is a real lifesaver!
Stats on the gear can be confusing the first time you encounter it. When you find items, examine them to see if they improve your gear significantly (hold your mouse over the gear. A popup will appear, showing you how much you will improve (or lose) if you use that gear.)
Here's my priority list for the Barbarian
3) Vitality OR Experience
4) Magic Pickup
Those are really sort of trivial (except for the first one.) Although a piece of armor may have a higher number than another piece, you may often find that because of other stats, the lower level armor is a better choice.
HIGHER LEVEL (remember, this is for Normal difficulty)
Sockets allow you to put gems in your gear. After my characters hit level 28 or so, I sell off any gear that isn't socketed (there's some things that don't come with sockets, but many do) and opt for socket and put red gems (strength) in them.
Some folks like to go for gear that gives them experience, which allows them to level quickly. It's not a bad choice, but unless there are other stats on that piece of armor that help my character, I ignore gear with extra experience after I hit level 10.
TWO HAND OR ONE HAND?
This depends on how you play the game. Having a great big shield adds to your armor but the tradeoff is that you pass up two-handed weapons specifically for Barbarians that give your damage a huge boost. If you have two items that are roughly the same (no improvement in stats), then look to see if one of them allows more attacks per second.
See the pretty red gems? Put them in your gear. Put them in ALL your gear for Normal difficulty.
Now, wasn't that easy?
Your buddies (by the way, you CAN retrain them so they have different skills in each section of the game.) As a Barbarian, I want to run in and smash things to tiny pieces. The Templar is a good smasher and can be set up as a healer as well -- but I'll be honest. I like the Scoundrel best. He has high damage (if you give him Dexterity gear) and his dialogue is more interesting. The Sorceress is annoying and the Templar is... moody. Most people set the Templar up as tank, Sorceress as crowd control, and the Rogue as DPS. If you find you're dying lots in one area, level up one level, change out some gear, and swap companions. The Templar was more useful for Belial on Normal than the Rogue was or the Sorceress.
By the way, a good practice is to STAY OUT OF THE GREEN STUFF. It inconveniences you and your followers on Normal level, but on the harder levels, it kills you and the followers very quickly.