You need to setup a maintenance schedule.
Lots of ways to do it, write your own, use maintenance plans, Ola's maintenance solution, Minion Ware, Red-Gate tools, Quest tools, Idera tools, ApexSQL tools.
Few links below for you to read up on transaction log management, backup & restore, maintenance plans, and my favorite Accidental DBA book helped me out so many times.
If it was me I'd go with Ola, but his scripts can be a little daunting, so read the maintenance plan book and go with that for a first pass.
How to post data/code for the best help - Jeff Moden[/url]
Need a string splitter, try this - Jeff Moden[/url]
How to post performance problems - Gail Shaw[/url]
Managing Transaction Logs - Gail Shaw[/url]
Troubleshooting SQL Server: A Guide for the Accidental DBA - Jonathan Kehayias and Ted Krueger[/url]