Do you know the species? I can’t quite make out what species it is, but it definitely looks like one that should be outside. Also, as mentioned, the soil that it’s currently living in looks like it’s holding too much water - a little research on a DIY bonsai soil will give you plenty of ways to create your own free draining soil using supplies that you can get in your area. Also, knowing where you are is always helpful to those offering help.
Where did you get the tree?Is this one that you collected yourself?