You will need to create an account in iStall first. See here for more details on creating an account.
If you have already created an account and verified your information, but are having trouble signing in, one of two things might be happening.
If you used Sign in with Google to create your iStall account, you need to ensure you are selecting this option at the login screen.
If you originally signed in with Google, the Password Reset button won't work, because you can only reset your Google Password through them. What is likely happening is that you are filling in your Google address and Password instead of clicking Sign in With Google. When you select this option, you see a popup that looks very similar to this:
If you used the Create New Account but simply can't remember your password, there is a convenient Password Reset button right below the Login prompt. (see below)
After selecting Password Reset, you'll be taken to a screen that prompts you to enter your email address:
After filling in your email address, click Send Reset Email and then check your email for a password reset link and/or reset code.