The first thing a good dba should do, coming into an existing environment, is to put together a backup scheme.  Typically, a full backup once a week, and transaction logs more often (depending on how your organization would view data loss in a DR scenario).  Secondly, maintenance plans, DBCC checks, reindexing, etc.  Some good ones to check out are DBCC CheckDB, DBCC Reindex, DBCC IndexDefrag, DBCC ShowContig.  Some of these are very intrusive and should be done off-hours.  Capacity planning is also important.  Monitor your db sizes and keep tabs on it.  Once you have all of these automated, then you can start getting your hands dirty with performance tuning at the query level, DTS, Replication.

Getting back to backups, be sure to test your backups on a regular basis.