CareerCup

take a piece of paper and pencil. start with the easiest ones. try to understand what each line does in the solved problems. also print every step in the code.
and go step by step
good start with Strings and Arrays

Realize that these coding questions are the worst kind for interviews. They only target your instant knowledge of algorithms and do nothing to indicate quality of code on a larger scale, ability to research through a problem, or how you work when faced with a task you cannot solve on your own. Anyone can learn how to count Fibonacci numbers, not everyone can understand why you'd want to bother counting them in the first place.

If companies you are interviewing with focus on generic coding questions like this, look somewhere else. These types of questions are some of the worst ways to hire people. I don't use them at all in my hiring process. These focus on your instant access knowledge to algorithms or generic techniques, and reveal very little about your problem solving skills, your ability to research and find solutions to complex problems, nor how you work with others. Instead, give examples of the work you've done and what impact it had.