Hey im making a low poly mountain scenery and need to make a lake or pond at the center of the plain how can i go about doing this and adding the water
Your question is extremely vague.
A vague answer: Add a plane and apply a water texture.
This may not be exactly what you want but you haven’t told us/shown us anything useful to explain what precisely you want.
When you ask a support question please at the very least make some king of effort or you’re wasting other peoples time as they try and tease some kind of info from you.