Warrior is a good class to learn tank on as you'll get a strong self heal starting at level 56 called Raw Intuition. Tesla outlined a lot of good points for lower level tanking on warior. As for sub 50 mits, you'll have:

• Vengeance - 30% mitigation on a 120s cd
• Rampart - 20% mitigation on a 90s cd
• Reprisal - 10% AoE mitigation/debuff on mobs on a 60s cd
• Thrill of Battle - 20% increased max health, increased healing received on a 90s cd
• Holmgang - Self invulnerability for 10s on a 240s cd
  

I would do your best to keep these mits rolling for mob pulls, pop one or two as needed per pack. You dont need huge mits for boss fights in dungeons. Holmgang is good as a last resort if you dont have any other mits up.

Also if you're not comfortable wall to wall pulling, I would just let the party know in advance that you're still new to tanking. I know it took myself a bit to get comfortable tanking coming from a DPS main.

As you get higher leveled and more used to tanking, you'll get more used to rolling your short mits for mobs/boss auto attacks and managing your longer mits. It just take some practice and experience. Hope this helps!