Don't listen to some of the people in this thread who don't have a clue what they're talking about, you don't get "insta-banned" for modding your Gamerscore, nor is there a high chance of a ban.

You only get banned if you have a prior history of modding and have been caught previously, first offences will gain you a gamerscore reset if caught.

There's always a chance of being caught, but using sensible discretion and using common sense will reduce the risk drastically.

Set your achievements/game library to private, only mod sensible amounts and not ridiculous amounts, and also use time-stamps.

Programs you can use are Horizon (the best tool), Velocity, Modio, 360 revolution and Le Fluffie.

Use the site's download section or Google to your discretion to find these. If you need further advice or help, my PM inbox is always open to you or anyone for that matter.