Can I lose weight by doing sports without dieting?
In short- yes.
Also in short, no.
So, its kinda both.
Yes, you could definitely lose weight by beginning to play sports if you are not an active person generally. If your diet stays exactly the same as it is now, but you add in additional exercise (such as playing sports), you will most likely lose some weight.
Maybe not a lot, but some.
If you only diet without exercise you also can lose weight, but eventually your progress will stall.
If you are going to be competing in sports, your diet should go along with your goals.
If you are strictly looking to lose weight, you should check out my article!
There are a lot of strategies for losing weight, but combining a consistent diet without proper exercise is easily the most natural way to do it.