To keep your Bonsai trees alive and in good shape it is important to fertilise them at the right time with the right fertiliser depending on the type of tree. You will find an enormous choice of fertiliser amongst bonsai shops and webshops. You can buy them as a liquid or in hard cubes. On every package you will find a code as in this example; NPK: 5:3:5 N stands for Nitrogen, P for Phosphorous and K for Potassium. The numbers are the ratio per element. In my opinion, fertilising still remains a very debatable and controversial topic in the industry.
Watering your Bonsai seems easy, but it's not. It depends on the kind of tree, which climate you are living in and what the weather says. It is also best to water your trees from above so that also the foliage absorbs water. Often they use a watering can with a turned neck to prevent that the waterflow is too strong and removes the akadama and earth from the pot. It is best to water your tree a couple of times so that water penetrates the soil and spreads evenly. During hot periods you might have to water your trees at least twice a day.
Protecting your Bonsai from heat is something, from cold is another thing. There are different ways of protecting your trees from frost during winter. The greatest error is to store them too warm and too much covered so that they aren't able to breath. This can result into mould development which can kill it. Best is to leave them in an aerated space between 5 and 15°C, with sufficient light. Another option is to leave them in a greenhouse.
Once you have some nice trees in your collection it might be nice to take some pictures or even to participate at an exhibition. If you are going to display them you could consider purchasing a bonsai display table. In Japan it was tradition to display a bonsai tree on such a table in a Tokonoma. If you are not bothered without too many wood carvings you could even make your own display table, as I did. When taking pictures make sure that the tree is at its best with suitable lighting and a corresponding background.