Maybe this will be unpopular, but it’s probably not worth it if it’s a really cheap suit. If you’re buying a suit, make sure it’s 100% wool (yes, even a summer suit). My top recommendation for affordable suits is Suit Supply, if you have one around you. I believe they will do many or most simple adjustments for free, which makes the price tag pretty hard to beat for a good quality suit. Regardless of what you get, a high quality suit will last much longer and be more cost efficient in the long run.
Edit: I’ve heard mixed things about Indochino and never shopped with them, so I can’t comment, but that is another option.
It’s also important to buy a fit that you like. Some people think tailoring will fix anything, but it can’t. If you want a slim fit, don’t buy a classic fit suit.
Unless it’s a really nice tailor, you shouldn’t need an appointment.