The first thing to say is to think carefully about the tailor you choose. First, find a good tailor in your area (check the site for that), then get a price for a suit, either including the material or without. Then before you commit yourself to a suit, make sure you go and have a chat with the tailor to make sure you’re comfortable with him and you are comfortable that he knows exactly what you require.
After that, we’ve produced a quick guide to see you through the process in an article called My First Suit, which should cover everything you want to know.
One thing worth noting though – although some tailors might ask for a deposit, the majority do not and no-one should ask you for the full money upfront. Basically, you don’t pay the lion’s share of your money until you are 100 per cent happy with the suit that’s been made. All good tailors are happy to make any alterations until you are happy – after all, it’s their reputation.