I just finished building my new wheel and I just realized that the hub isn’t centered. I thought it was gonna center itself when tentionning the spokes but it didn’t happen. Help, why did that happen?
EDIT: My rim looks beaten up, but it’s not, it really just is the hub that isn’t centered
The wheel is dished. To fix it go around the wheel and loosen the left-side spokes a half-turn while tightening the right-side spokes a half-turn. Repeat as needed.
After you lace it you have to bring up the tension a bit, and then dish the wheel. Dishing the wheel is centering the rim between the bearings. Once the dish is good, you continue to tension the wheel. When the wheel is just about up to tension you true it, and dish it again. At this point you are into final stressing/tensioning/truing. When you get confident in your wheelbuilding you will do some of the re-dishing along with truing and tensioning. You get a feel for it after awhile. If you have a calibrated Park TS-2 truing stand it will show you the dish at the same time that you are tensioning and truing, so it can make it a bit faster. Most of the TS-2’s I’ve used haven’t been calibrated though.
To fix it, I would go around the side that is further from the frame, and give it a half turn tighter for each nipple. That should move the rim to that side. My assumption is that you are probably not up to tension yet. This is based mostly on the many first wheelbuilds that I have come across. If it ends up being too much it would probably be alright to go around the other side with a quarter turn on each nipple. When the tension seems like it’s right you need to work more subtly by tightening, and loosening on the appropriate sides as you near completion.
Hopefully this answers more questions than it creates. Take a look at Sheldon Browns wheel page. I understand it’s helpful.
Right advice, but the wheel is not dished, that’s the problem.
Or move the camera a little to the right, and it will be lined up perfectly! 
With the wheel not attached to anything, it’s hard to tell how much dish there is. Make sure you’re correcting only the actual dishing that’s there, and not any extra that you may be imagining. Most unicycle frames work well as poor-man’s truing stands. Uh, I’m not a poor man, but I don’t have a truing stand either. If you don’t have one, use the frame. But some frames may be a little off also, so put the wheel in both ways to see if it stays centered first.
If you want to be pedantic, the term “dished” comes from “shaped like a dish” (just like taco’ed comes from “shaped like a taco”). You don’t want your wheel to be shaped like a dish.
Oops, typo in my last post. I meant, move the camera to the left to fix the dishing.
When jtrops says Dishing, it’s probably standard bike terminology for truing a rear wheel. With front wheels (or unicycle wheels) you probably want to make sure they’re un-dished, or centered.
This is very important, because a unicycle wheel with any offset will tend to pull to one side. When I rebuilt my first 36’er wheel, I experimented with wheel dishing to counteract the typical road camber I see here in LA… and discovered that a 2mm dish to the right did a very good job of that. Of course, if you’re a Brit, then that would be a dish to the left
Okay then… here’s something I call the “poor mans dishing tool”. If you’ve seen a dishing tool before and tried to buy one, you already know that they are quite expensive if you’re going to only use them for those rare moments that you’re working on your own wheel. Besides, in my case, I discovered that nobody makes one that is compatible with a 36" wheel!
So… 4-6 rubber bands, a couple of sticks a few inches long (any kind) and a STRAIGHT piece of wood as long or longer than your wheel diameter.
Use the rubber bands to wind around and hold the two sticks at right angles to the straight piece of wood, about where the rim should be. You now have one long piece of wood with a couple of adjustable short sticks.
Lay the straight piece of wood against a good point of reference, your bearing or the end of your crank/hub axle, and adjust the positions of the two sticks until they just BARELY touch the rim on both sides. Make sure that you have three points of contact… the rim on each side plus your point of reference.
Now lift the wood away and flip the wheel over, lay the piece of wood against the same point of reference on this side and see if it matches. You will be able to clearly tell which way the rim should move, which is the side that you want to tighten towards (and loosen the other side).
RE-do this dishing test as needed till you get the wheel centered.
Having build probably 1,000 wheelsets in my bike shop days, this is the way I do it now 
The quality of the result is far more important than the cost of the tool.
Oh, and one other point… if you leave the wheel in the frame but remove the cranks, you can use the end of the crank/hub axle as your point of reference while still keeping your wheel in it’s natural truing stand.
Thanks all.
After loosening all my spokes a just centering my hub by moving it with my hands, I realised all the spokes on one side were “too long”. I think I just need to offset all my spokes from one hole in the hub flange.
Yup, that was the problem.