What type of tree is it? Many trees can be trained into bonsai from maples to pines. Sticky leaves can mean the residue of aphid or other insects. You could try wiping each leaf with an insecticidal soap solution or just using a strong jet of water to clean the leaves. Use a magnifying glass to try and properly identify the pest before deciding on a treatment. Perhaps if you sent a photo of the tree and a closeup of the leaves I could be a better help. Keep growing, Marianne Binetti