>>12462373algebra, algebra, algebra. Get good at algebra. Have a rock solid foundation in algebra.
Currently working on a computer engineering BS. The highest level math I've taken so far is differential equations (this, after taking calc 1-3). So far all of the math I have taken invariably uses algebra. If you're good at algebra, you have a good chance at being a solid mathematician.
Geometry and trig are also good foundations, but algebra is by far the most important. Its importance can't be overstated. Learning algebra for anything math related is like learning how to read for anything literature related.