But the general rule of thumb is that you should pay no more than 35% of your gross income towards your home. So if you earn $30K per year - your annual mortgage should be no more than $10.5K per year or $875.00 per month.
I agree with Cindy, you should talk to a mortgage broker to find out how much house you can afford. The reason is because this is not straight forward - how much monthly payment you can afford to pay depends a lot on the amount you are borrowing (not in order of importance): :
How much down payment / closing cost you have, points you are willing to pay, the credit rating you have (which affects the interest rate), property tax, insurance, other debts you might have (lenders will be interested in finding out your debt to income ratio), the kind of loan you are getting (adjustable rate, fixed rate, interest only,, etc), your income potential, how long you have been on the job, whether you are a salaried employee or not, etc.
Each answer will have some effect on how much money you can borrow from the lender, and thus how much money you can pay for a house.
A good, reputable mortgage broker will sit down with you and go through the numbers and difference options you might have. They will also give you suggestions on what to work on to improve your options.
Your friend, family or Realtor can recommend good mortgage brokers to you to get started.