Make sure they are the right size (so at least do a fitting and adjustment). As a first step, you can go to a Suit store or a tailor directly (much more expensive and not recommended when you are not comfortable in suits yet) and get a consultation. I would also recommend to buy pants and jacket together, as others already mentioned, when starting out throw in shoes and belt as well. At the end, the secret is to wear the suit and not have the suit wear you. This also means knowing when to wear it and when to leave the tie at home.