Hello ive not been playing long, i can play a few simple songs using tabs and a few scales etc, all i want to know is when you start jaming along with guitarists how do you make a good bass line, say if the guitarist is playing a d chord i can obviously play the d note on my bass, but i want to know how to make it sound better and abit more complicated and what other notes i can be using to fit in with that guitarists D chord, i hope sombody understands what im trying to say and replies, thanks

... how do you make a good bass line...

A question that we are all attempting to answer.
For playing in D: http://en.wikipedia.org/wiki/D_major
Free online lessons: http://www.talkbass.com/forum/showthread.php?t=481548

Here's a few more TB threads that may help you along on your path to bass wisdom:

http://www.talkbass.com/forum/showthread.php?t=434246 Best musical advice
http://www.talkbass.com/forum/showthread.php?t=479401 Learn to read music
http://www.talkbass.com/forum/showthread.php?t=479642 Learn to read music
http://www.talkbass.com/forum/showthread.php?t=419687 Read bass clef
http://www.talkbass.com/forum/showthread.php?t=473968 Theory
http://www.talkbass.com/forum/showthread.php?t=481465 Things every bassist should know
http://www.talkbass.com/forum/showthread.php?t=99676 Theory Book Links
http://www.talkbass.com/forum/showthread.php?t=56761 GI links list
http://www.talkbass.com/forum/showthread.php?t=482544 Jazz Scales
http://www.talkbass.com/forum/showpost.php?p=6485459&postcount=37 Understanding a bass player's mindset
http://www.talkbass.com/forum/showthread.php?t=484065 Guitar players trying to play bass

Online study of music theory/ear training.
http://www.teoria.com/ Check out the tutorials, exercises and reference(dictionary).
http://www.good-ear.com/
http://www.studybass.com/
http://www.activebass.com/

Books and instructional materials
http://www.bassbooks.com/shopping/default.asp

Free Jam tracks
http://www.freejamtracks.com/index.html

This is a very helpful program for learning the notes on the bass clef. (as recommended by AlphaMale)
http://www.download.com/NoteCard/3000-2133_4-10004005.html?tag=mncol&cdlPid=1509909