Assuming you are a beginner trader, and is now ready to take a lot of risk, you could start with a 5 over 20 ema crossover for short-term trading.
As you gain experience you will realize that higher the ema's for crossover the signal gets better.
After a few months, you will automatically be looking for 20 over 50 ema crossover.
After a few years have passed, now that you have witnessed a few market cycles, you are now looking for 100 over 200 ema crossovers.
This is a process for improvising yourself into pro trader. One thing to remember, do not loose all your chips too early.