How bad is purging also known as self induced vomiting?

Never purge unless instructed by a doctor to do so (if you have swallowed something poisonous accidentally, etc).

Purging regularly - as in the illness bulimia - is extremely dangerous. It can destroy the human digestive system (meaning you will never be able to eat "normally" again and might face being fed through a tube the rest of your life), strip the enamel off teeth (stomach acid is part of what comes up), and sometimes end in death, often after a person is finally healed/in recovery from bulimia, see the life of the late Karen Carpenter.

The human body needs fuel to run properly, and we find fuel in regular meals which give nutrients to the body, then the body gets rid of the waste naturally through urine and bowel movements.

Please see your doctor, and if you are looking for ways to lose weight, your doctor can help you in that area as well. Or check a site like livestrong, they have tools that show you how many calories you need to eat daily to have and maintain a healthy weight.