Should I go for fixed hp?

[Sergio]

I was rolling my hp dice for my barbarian for reaching level 6

we get max hp at level 1 as house rule. So 12 hp first

(d12) Then I rolled the first time: 7
(d12)then a second time: 1
(d12) then a third time: 1
then a forth time with a d10: 6

Should I go for average rolls, next time?

[Aegis013]

I would say it depends on what you want.
If you want to have as much HP as possible, I would say go for average rolls IF average is considered to be rounded up (7 on a d12) or true average (6.5 on a d12).
Go for rolls if your table rounds down (6 on a d12).

If you don't mind the craziness of rolls, they can be fun, but as you've seen they can be problematic.
My personal houserule for this is to have HD be roll 1/2HD+1/2HD (so D12 is d6+6). This keeps some random element to the HP rolls, but ensures that aberrations like the 16 Con Barbarian with less health than the 14 Con Wizard simply can't happen.

[frogglesmash]

FYI, getting max HP at 1st level is not a house rule.

[Rijan_Sai]

FYI, getting max HP at 1st level is not a house rule.

This, also the general rules for average HP (IIRC) follow a "Odd high/Even low" pattern.
That is, with your HD as the example:
Level 1 = 12
L2 = 6
L3 = 7
L4 = 6
L5 = 6 (since you had a D10 as your fourth roll, which would be level 5, the average is 5.5, and all odd numbered levels are "high".)

[Âmesang]

I typically go with true average results* …mainly 'cause I'm lazy. Also in the off chance the character is level-drained it makes calculation easier.

*As such I tend to also go average with the spell, false life, rounding up or down depending on the mathematical result of the hit points.

[Elder_Basilisk]

I'd recommend going with average. My experience is that getting above average rolls doesn't help you as much as getting below average rolls hurts you.

[Jormengand]

My experience is that getting above average rolls doesn't help you as much as getting below average rolls hurts you.

This is also written in the DMG, a rare case of Wizards understanding how their own game works.