This is not a subject for the faint of heart, and it's not something you can just "cookbook" on a forum web site.
There are several different methods. You can use Replication (the "Publish/Subscribe" model), you can use linked servers, you can do log-shipping, you can use changed-data-capture with periodic updates (with DTS or SSIS depending on where you want it to run). Depending on how fancy you want to get and how much you want to spend, there's always mirroring and clustering.
If you are new to the subject area, you should probably read the manual's "Replication" section to see if that fits your need. It's the simplest to implement, has the fewest security headaches and isn't particularly a resource hog.
Good luck!
