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Check the side of your tire - the size is written there, like 26", 27.5" or 29". It has to match the rim size of your wheel, because you can't just slap on a different size. If you're not sure, check your bike's manual or ask at the bike shop - they know their stuff and can recommend the right width for you too.
The rim size is really the most important thing - if your wheel is designed for 27.5", for example, only tires in that size will fit on it. Anything else leads to problems with the brakes, the frame, or simply because it won't fit mechanically. Best thing is to check your bike's manual or ask at the shop where you bought it - they can tell you the exact size.
With me, I once bought a tire blind because I didn't check the size. Turned out my old hardtail was 26" and I grabbed 29" - of course nothing fit. Now I always write down the size or take a photo of the current tire before I head out.
Besides the inch size, there's also the width - how thick the tire is - you can see that on the tire sidewall too (like 27.5x2.25). If you want to run something wider, your frame and rim have to support it, otherwise it'll rub somewhere or the tire will fall out. Some bikes are more flexible with that, others not so much.
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