There are a few ways you can customise your character. One is by spending vouchers, to spend your vouchers you need to go to the 'shop' which is based in the home town and click on barter, then vouchers. There you will find items which will allow you to customise your character.
Another way is by purchasing coins which is done by converting Ap to coins. This means spending real money, but it comes with a few advantages. When you spend coins you can make items permanent so you don't have to worry about them expiring and also you have a much wider range of items to choose from.
Note that you should use your vouchers efficiently because they aren't very easy to collect and if you are looking to upkeep all your items make sure to do your daily tasks regularly.