The first step you should do is to check your savings. Figure out how much cash you've got left after essentials, bills and other requirements. You might want to stick your spare cash in one big lump sum, or you might fancy setting up a regular monthly payment. Ask yourself:
"How much can I afford to invest?"
Answer these biggies and you should be able to determine how long to invest for and how much risk you're willing and able to take.