The term 'bonsai' simply means "plant in a tray (or) pot'... so for comprehensive advice is difficult not knowing the species of your 'plant in a tray'. Assuming it's the inveterate small fir, such as a juniper... simply keep it well watered. Watering the base of the plant isn't sufficient, however and it should be "spritzed" at least daily, more often in the winter months when the humidity is low. Once a year, I take mine (I have several) completely out of the tray and comb out the roots. I then remove about 1/4 of them randomly to maintain the small size. Replensih the soil and re-plant. Don't over fertilize and watch for mites. They will appear as pencil dot size scampering across a white sheet of paper that you hold under the branches and shake lightly. Treat as recommended by your plant store if they appear. They'll kill it inshort time otherwise...