Which level do I start arena at?

[gojo satoryu]

I tried arena on lvl 5 but I keep getting killed very fast

[Bomb Bloke]

The champion is merely level 10, so I suppose that's what I'd suggest.

[Bansheebutt]

Pretty sure the Arena is scaled to your level like virtually everything else in the game.

[Bomb Bloke]

That particular NPC is locked, though it's true that the unofficial patch changes this.

[gojo satoryu]

I have unofficial patch

[Bomb Bloke]

In that case, he's locked to your level plus ten, in which case it's less about what level you are and more about what abilities you have.

If you haven't already, it's a good idea to join the mage's guild (that is, get all the recommendations so you can enter the guild site proper at the Imperial City). Even if you're not interested in "being a mage", any adventurer worth their salt at least has a few utility spells to sling around. Summons to act as a distraction, for example.

[gojo satoryu]

I am a mage but my infamy is 5 and I am in thieves guild,so I cant join

[Bomb Bloke]

Those two factors are irrelevant. You should be able to join so long as you don't have a bounty.

That said, it is possible to reset your infamy[en.uesp.net], if you so desire.