I have adobe premiere 2018, but i dont know how to pay for it

check your account to see if you already have a plan, https://account.adobe.com

if you see a trail plan, it will automatically be converted to a paid plan.  make sure it's the one you want.  (there are 3 options for premiere pro subscription, annual paid once per year, annual paid monthly and monthly paid monthly).

but also make sure you're using adobe premiere pro as opposed to premiere elements.  premiere elements is not a subscription program.  it's a one time payment for a serialized app.