To give you an answer is pretty tough without more information.
Are you using a Windows Server or Unix?
Do you need the completed form saved to a database?
What Theme are you using, perhaps it has a built in form system that you can modify and add a PayPal stack?
Are you ever going to change the Theme (This could change the form functionality if using the Theme’s)?
Do you want form content validation on any fields, other than required?
Any additional information you require from member other than name, address, phone number, email address?
Did you want to issue a member number or use the receipt number as their membership number?
Did you want them to have the ability to download a printable membership certificate or card with their name and membership number on it for digital delivery?
You are only ever going to use PayPal and never a CC or Debit Card or other payment methods like cheque, money order?
You will find the answers vary greatly from people if you don’t provide this sort of information when asking such a question.
You can check out the eCommerce solutions on the RapidWeaver Community website, you can do a search for forms, you can do a search for Paypal and you will find a bunch of Plugins & Stacks.
Hope that helps a bit…