Work out your essential outgoings - housing, food, bills etc, in order to work out how much money you can invest without leaving you short of money. Stick to this amount. Always bear in mind that in a worse-case scenario you may lose all your money. Stock prices can go down as well as up so never depend on the money you're investing.
As you become more familiar with the stock exchange, you may wish to start investing more, or re-investing any profits you may have made. As long as you keep in mind that the stock market is ever-changing, you'll make the right decision for you. Some people invest constantly, others may make a profit and quit while they're ahead.
