Suit alterations are very common. Very few suits will fit well off the rack. Its important to know what can be altered easily and what can't.
General off the rack suits tend to be a little bit boxy and square. This is especially true in the US, where the jackets especially can be big in the waist area and long on the sleeves. I guess we are big guys! Very few will look perfect and be ready to wear out of the store.
This isn't just the case for new clothes though. You may find that what you just bought a couple years ago doesn't quite fit. Big weightlifting will punch up your shoulders and too much beer will definitely kick up the gut. If your body style has changed it might be worth spending a few bucks to get some work done.
Common suit alterations fall into three categories: sleeves, jacket waist, and trouser cuffs.
Jackets bought off the rack generally have sleeves that are too long. This can make you look like you are swimming in your suit. You should have the sleeve go to just where the wrist meets the hand, or at the wristbone knob. This will give enough room for your shirt to hang and be visible.
The jacket waist is a little tougher to gauge. Most suitcoats will be somewhat shapeless and won't conture to your body correctly. You want to slim the waist a little bit to help emphasize your chest and shoulders, without padding those parts. You'll have to work with a tailor on this, but just know that the jacket should feel snug but not tight. The buttons shouldn't be straining to hold on for dear life!
Trouser length are a nearly universal alteration. Most pants will come unfinished and be way to long for most men. Most men should go for cuffs. Non-cuffs only really work for skinny, non-pleated pants. Most tailors can show you a little bit about what the break will be before you get it. If you go for cuffs, here are some of the options you have:
Traditional, or full, break: this goes down to the heel's top and covers most of the laces. A good choice in most cases.
Half-break: This goes down about halfway down the back of the shoe, and thus covers less of the laces. It works with most pants types but may show some sock when you walk.
No break: Not recommended for most suits. However, a lot of hipsters, mod suits, and 60s fanatics might like this type. Your socks will be sticking out there for all to see though!