Every contest has its own specific so I would recommend to focus on previous editions of it. For SPOJ you can find tasks from previous editions. You should check them first.
Regarding time its good to try to solve easier problem first. Its a good warm up, its has motivational aspect and allows you to get al least some points (depending on the score system).
Regarding hidden features, hard to say. SPOJ is quite cruel and doesnt give you to much insight about your invalid submission. Good to know that you can assert some condition in your code and you will see that exception in the submission result. This is good way to check if your code meets specific scenario (which you didnt cover for example).
