Gaviao,
There's no indication in your profile of where you might be located to this advice may be a bit off target, but...
If you really have no idea what fits you, not unlike me not that long ago (and I'm still learning as I go), why not go to some local thrift stores and look for suits in 42 short. Goodwill and The Salvation Army usually sell them for less than $20.
If possible but not absolutely necessary, try to find suits in navy or gray WOOL. Avoid acrylics (poylester, etc) and silk for now. Pinstripes (north to south, only!) are okay but be wary of patterns until you learn what works on your body shape.
There's a whole piece you will learn involving canvassed suits vs. fused linings but that won't matter much right now except to say that canvassed is considered better in most instances. Do a search to find out how to determine if a suit is canvassed.
Look in the mirror when you try the suits on and study how the shoulders fit you. You likely don't want anything heavily padded and they should not extend beyond your natural shoulder. This is a VERY hard concept to grasp initially as (if you American), we tend to pay little attention to how the shoulders fit and seem to be mostly concerned about the chest and the gut. Those things can be fixed fairly inexpensively but big shoulders will be a deal breaker when it comes time to have any alterations done. They're not cost effective for most suits. Obviously, there may be exceptions to this.
Next, look at the lapel notches. You can learn about the various lapel styles elsewhere on the forum but just make sure they don't sit too low on your chest. Currently, the higher and closer to your collarbone they are, the better.
Never mind about sleeves that are too long but be careful if they are too short though that doesn't sound likely given your measurements. Sleeves are easy to fix.
There are a variety of opinions on jacket length but basically,
CYA (definition #2). If your arms are hanging straight down and the jacket hangs below your downward pointed thumbs, it's too long. How much higher you want it to be is a matter of opinion as well but just remember to CYA.
Once you've found a suit that fits these needs, pay the nice lady at the checkout and take the suit to a local big box suit retailer like K&G or Syms with an alterations department that accepts clothing that DOES NOT HAVE TO BE PURCHASED AT THAT STORE.
- Have them adjust the sleeve length to show a little shirt cuff, maybe 1/8 to 1/4 of an inch.
- Ask for very little or no break in the pants length. Let them take in the crotch to remove any excess fabric in the seat of the pants. You can even have the legs tapered to give you a slimmer look. Bonus!
- Have them adjust the fit of the coat. They can take in the back from the center seam. They can suppress the waist a bit from the side seams. This will probably feel alien to you. If the jacket can be made to feel like it is gently caressing you rather than hanging far away from your body, you're in the ball park.
If you find someone reasonable and bought well, you should come out quite presentable and far ahead of most of the world for under a hundred bucks. Apply lessons learned with this starter suit to better/newer/more expensive finds as you build your wardrobe.
Good luck and happy hunting.