How to start minecraft beta?

If you are asking how to load the game…

Go to Minecraft.net, make an account, and purchase the game. Now you can log on to your account and play in browser, or click the "download" link on the homepage to download the client. You can click single player to play single player by creating your own map (leave the seed field blank if you don't know what it means). To play multiplayer, you will have to find a host's IP and type it in to join their game. Some server may have whitelists, so you can't join unless you apply to their server (if allowed), wherever you found their server IP. A good way to find servers of all sorts is to go to the minecraft forums page and click Beta Survival Servers and read the threads.

If you are asking how to start playing and what to do…

Go to minecraft.net and click the help link and watch the tutorial videos there. They will help you.