1. First of all, determine whether you should be investing at all. If you have any debt other than a home mortgage your first priority should be paying off that debt. It doesn't make sense to pay more in interest than you earn on your investments. The only exception to this is investing for retirement or education in a tax sheltered account like an IRA, 401K or 529 plan. Since you are getting a tax benefit it always makes sense to invest in them if you can spare the money.
2. A good investor always pays himself or herself first. Examine your monthly budget. After you take out what you need for essentials like food, shelter and utilities how much is left? Decide what you can afford and invest that much every month just like it is another bill. If you get a raise or a bonus invest it as if you never laid your hands on it. Also, avoid debt at all cost. Don't buy things you can't pay for immediately. Debt is the opposite of investing.