Build a wallet for Android and iOS for a self-made cryptocurrency
You first should learn how to create an android app if you don't know how to do that already. There are lots of resources available for this and better places to ask questions than the bitcoin stackexchange.

Then once you are comfortable with that most android wallets are built using bitcoinj. I don't know of any nice step-by-step guide. Here is a sample github project for bitcoin that should give you a good starting point. https://github.com/thinkmobiles/BitcoinJ-Wallet-Sample-Android. I don't know anything about your coin but you will obviously need to make some fairly critical changes.