LeftyBassist.com

this might sound simple but, just listen to drummer, just play something simple to go along with it, if youre just starting out don't feel as if you have to __________ to make it musical, as long as you keep the tempo you'll be good, what i did with my friends when we all first started playing was the "green light rule" as long as you keep the tempo you could play whatever you wanted, you could be out of key ultra busy, super laid back, as long as you kept tempo, it makes things intresting and after 10 min or so all this random noise occuring it will all gel together and it will teach what works and what dosent work

No wich is kinda of wierd

i use the old Ox trick
I remember him talking about the who days and how he would play a simple riff over and over, then after awhile change it to another

This is how i start if Im jamming or stuffing around.

Start with a riff, eg taxman , has a good formation to build on.
Or a blues lick eg roadhouse blues etc
then do slight variations on top of that riff until the original riff is gone.
play these riffs for 4, 8 ,12 or more bars, this allows the drummer to lock in (as you are playing the same riff) also it gives you time to think up another riff in your head

good enough for The Ox good enough for me