Hi,

My recommendation would be the following:
Allen keys: 2, 2.5, 3, 4, 5, 6 and 8mm
Tire levers
Spare tube (I don't usually bother with on-trail patching but the choice is up to you )
Pump
Chain tool (A chain link may pop open anytime, or some links may die during crash carnage and you'll need a shortening of the chain to cycle home)
Spoke tool - possibly, I carry it just in case a spoke comes loose or something, it's not too heavy so I figure what the hell, might as well take it with me.

This is just the basics that I take, then there's the typical "lots of water, lots of food, camera, jacket, etc" but they don't fit under the "tools" description. What I'd recommend is getting a home toolkit (I got mine for 40 euros) which, while cheap, actually has decent tools which allow you to do pretty much any repair or installation on your bike except headset pressing. This way you can spare loads of cash and time by not having to go to the bikeshop and you'll have fun on your way to becoming a home mechanic.