The 4-H market sale is one of the lessons of the whole project. I am not aware of anywhere that the 4-H market animals aren't auctioned off. Here is the quick & dirty outine of a 4-H market steer project.
1. Buy steer in late fall or winter & halterbreak
2. Weigh in early spring (for the Western MT Fair weigh-in is Jan 8th)
3. Feed critter all spring & summer (rations & amounts vary depending on initial weight and target "finishing" weight)
4. While feeding also train him to lead, be brushed, "set up" and be handled and or touched everywhere (be careful), practice fitting him and get him used to show halter, show stick (for "setting up") also wash him and get him used to buckets, etc that you will use at fair.
5. At fair, they have the weigh-in for calculating weight gain and auction prices. You'll show the steer, usually broken down into weight catagories and then all purples show against each other for Grand & Reserve Champ. There may even be showmanship competitions. Then at the market animal auction all the market animals from rabbit to beef are auctioned off so that you may learn about the financial end of the project. Here if you get a red ribbon you cannot sell your steer in the auction. Prices are generally much better than market to support the local 4-Hers who put so much work into their animals.
6. Animals may be eligable for carcass evaluation and awards depending on how it is purchased and how your state is set up.
I can get into way more detail about the auction etc. but each sale may be slightly different.
As for breed - Angus, red Angus & Herefords are always popular, An AngusxHereford cross should do well, Limos, Simmental & Maine Anjou or crosses should do well. BUT feed and exercise can make or break you - too little or too much makes a big difference. It might not be a bad idea to find out what did well last year at the fair both in weight gain, ribbon competition and carcass results - the more you learn now the better off you'll be.