First you'll need an RJ45 Crossover cable to connect laptop with xbox. It looks exactly the same as normal network cable but differently wired. You could either buy it of if you have set of crimps, make one yourself. Once you have the cable do a google search on Internet Connection Sharing with Ubuntu. There are lots of guides on this.
If you were 5 I'd tell you not to play with electronic equipment unsupervised and ask the parents