How to get a mortgage ?

You should definitely contact the professionals.

First things first, You need to document your tips meticulously and put all that income on your tax return. A few years of steady tax returns is enough for some lenders.
There are LOTS of lenders out there, so keep shopping around until you find someone who wants your business. A close friend of mine makes use of Fairway Mortgage Company, so look them up. All commercial lenders have to make the paperwork conform to specific standards, so nobody is going to loan on a wing and a prayer, but tax return history is an acceptable method.
Most people who make a “lot of tips” don’t keep very good records, and when they do, they realize they have never made as much in tips as they thought they did… For a few people it works the other way, and once they have accurate records they owe more taxes.