Jenna,

post a picture if you can. That would help with the advice.
Aloha

Jenna,

Congrats! I'm excited for you. Its important to know what tree you have in order to properly care for it. Can you post a picture for ID?

My guess is the soil its in is the soil you bought the plant in? What is it like? Is it like your everyday commercial potting mix? If so, consider waiting until the spring to repot into proper bonsai soil. Bonsai soil si free draining and coarse. Generally speaking, that is something to the effect of:

50% turface, perlite, lava rock, etc. (you pick one)
50% pine or fir bark, etc.

If you have a slow draining soil currently, be very careful with how frequently you water your plant. You may kill it, otherwise. You should water it when it is still damp and not quite yet dry. Drought and overwatering both take a toll on the plant. You can check for dryness with a chopstick or toothpick. Penetrate the soil through to the floor of the pot and leave it there for 15 minutes. If there is little to no dampness, you should water the plant. If it is wet, well then don't water it. Feed once a week with fertilizer.

Leaves say a lot about how your tree is doing. Yellow leaves may indicate too much water, while browning leaves indicate drought.

Make sure your pot allows for draining - are there holes underneath?

Like I said, how you care for a bonsai varies from species to species... so whatever that says trumps what I've said. So, you should get your tree identified.

Good luck!

Cheers,
Adam