I can't seem to get past the first "lo-rider" mission where you have
to bounce the car to the beat of some music.

Any ideas how to do this?

Everything you need should be here: http://www.gtasanandreas.net/walkthrough/

From the above site:

Once again, Sweet is arguing with his sister Kendl. Sweet doesn't like
the fact that Kendl is dating Cesar of the Varios Los Aztecas, a rival
gang. Sweet wants you to follow Kendl and check out Cesar, to make
sure that she isn't making a huge mistake. When you have control over
CJ, goto the garage in Willowfield. Sweet called ahead and asked his
friend [who owes Sweet big time because of a past incident] to give CJ
a car that bounces. Whilst in the car, backup into the garage. You now
have a chance to modify your ride, although it isn't absolutely
necessary. Press L3 to lock the car's hydraulics in place. Move the
right analog stick to control the hydraulic suspension and press R3 to
make it hop. Get to the lowrider meeting at Unity Station, El Corona.
Drive into the red marker. You have 3 options: Increase Wager,
Decrease Wager and Proceed. The minimum wager is $50 and the maximum
is $1000. If you lose this contest, the money that you are betted will
be lost to the opposition.

A series of directions will be shown across the screen. You must move
the right analog stick in the direction shown, exactly as it passes
through the circle on the middle of the screen. Try and not let the
music put you off -- concentrate on the circle. For every bad move,
points are added to the opposition. After successfully winning this
contest, a short cutscene will take place. Kendl is delighted with
your performance in the lowrider challenge, and Cesar introduces
himself. Carl demands that he take his hands off his sister, but after
some discussion, he agrees to give Cesar a chance. One of the Varios
boys tries to pick a fight with CJ, but Cesar holds him off.

That's IT - thanks!