The first time you open StashApp you will need to create a wallet, you will be greeted with the screen as in the image below:
Public key: this is your wallet address, it can safely be shared with others to receive funds to your wallet.
Private key: Safely back this up and never share this with anyone. This is needed to restore your wallet should you lose access to the device, reinstall Stashapp, etc. It can be retrieved later in settings, see here for more details.
If you have a referal code enter it and then tap on Proceed to create the wallet. Once created, you will need to proceed to activate the wallet.