Watch the Samba Basic online guitar lesson by Muriel Anderson from 50 Right-Hand Techniques You Must Know
Samba is a beautiful style on the guitar, and a whole world unto itself. Here is a basic Samba pattern that can get you used to the rhythm.
Then you can improvise on the notes and the patterns. A lot of the notes will fall on off-beats. It's also nice to use "ghost notes" in samba rhythms, and allow some of the notes to ring longer while other notes are cut shorter.