It'll just be teething problems with executing the movements. Try Ken's Ultra 1, two quarter-circle-forwards with all three punch buttons when you're hit the last forward (or your PPP-mapped button). If you can do fireballs, then you can do that Ultra. The two quarter-circles don't need to be stupid fast (because you'll likely fluff the inputs and not stupid slow, just smooth and steady, boop-boop-press. Blanka's Ultra 1 (Charge back, then quickly forward, back, forward+3xPunches) is a little less intuitive but will still come out readily enough.
One thing to check if you're genuinely confused about why nothing is happening: in training mode have switched your Revenge Meter to full? Is that thing the corner all fiery and flashy?
For combos, get started on the trials in challenge mode. Keep plugging through them until you hit a brick wall. With most of the cast you should be able to get up to the early/mid-teens with little problem outside of a bit of perseverence and you'll pick up some basic combo action that'll help your timing.