You need to feed your bonsai tree during the growing period of the year – which starts about now!
There are a few special bonsai feeds available, all are expensive, and they really are no better than general plant food.
For non flowering tree species we use Vitafeed 301 which is high in nitrogen and especially for leafy plants. This is a good feed, especially at this time of year when you want to encourage leaf growth. Ideally use this feed from now until the beginning of August when you should switch to Vitafeeds 111 which is a more balanced feed that will build up your trees strength ready for the winter.
For flowering trees switch to 111 earlier, around the end of May. 111 will encourage the growth of flowers.
Vitafeed is available from most garden centres or Amazon.