You must have a PayPal Business Account.
For live transactions, you must enable SSL / https for your domain. 
If you use PayPal – use currencies supported by PayPal

Create the PayPal processor page in admin theme pages

The page itself is not a link that should be accessed directly. The API will use it to communicate with the theme and the merchant. You will see a generic message when opening the URL in the browser. It’s correct and there is no issue with the page.
Have only 1 page with this template

You can add SANDBOX (for test) or LIVE credentials.

API details will be different based on your selection.

Add the LIVE or the Sandbox API in Paypal settings

How to take your Sandbox or LIVE credentials

1.Login at https://developer.paypal.com to create the API

2. Go to Dashboard Menu

 

3. Optional: How to create a sandbox account

If you wish to test with sandbox mode (all transactions are purely fictional) you must create your own business sandbox account from Sandbox – Accounts and select it when you add a new app.

 

4. Go to My APPS to create the new app:

Type the APP name and select the sandbox email (will be useful if you wish to test sandbox mode before doing live transactions)

Add webhook

 

5. Copy the Client ID and Secret id.

Be careful when copy SANDBOX or LIVE. You must set the theme options to match Sandbox or Live too. See STEP3.

OPTIONAL: For Rentals Club PayPal Payments Add-on

Activate PAYOUTS your API setup in order to be able to send money and check with PayPal if your account is elligible for PayPal Payouts processing

More on PayPal Adaptive Payments: https://developer.paypal.com/docs/classic/adaptive-payments/integration-guide/APIntro/

Select if you wish to use Sandbox (a test environment) or Live (real transactions)

Enable PayPal and Add the API Details

Don’t forget to add your paypal email to receive payments

Add your sandbox email to test payments or your live paypal for real payments.

Add the PayPal Processor URL in your PayPal account for recurring payments (with membership mode enabled)

For sandbox tests, you must go to https://www.sandbox.paypal.com/, login with the sandbox account you created above

For live tests, go to https://www.paypal.com

PayPal common errors:

error:14077410:SSL%20routines:SSL23_GET_SERVER_HELLO:sslv3%20alert%20handshake%20failure

Please see this url with a similar issue http://stackoverflow.com/questions/34926940/php-paypal-error-14077410ssl-routinesssl23-get-server-hellosslv3-alert-hands

And the solution:

Support SHA-256. PayPal is upgrading SSL certificates on all Live and Sandbox endpoints from SHA-1 to the stronger and more robust SHA-256 algorithm. You will need to update your integration to support certificates using SHA-256.

Discontinue use of the VeriSign G2 Root Certificate. In accordance with industry standards, PayPal will no longer honor secure connections that require the VeriSign G2 Root Certificate for trust validation. Only secure connection requests that are expecting our certificate/trust chain to be signed by the G5 Root Certificate will result in successful secure connections.

This document has the details to fwd to hosting:

https://www.paypal-knowledge.com/resources/sites/PAYPAL/content/live/FAQ/1000/FAQ1766/en_US/2015%20Merchant%20Security%20System%20Upgrade%20Guide%20%28U.S.%20English%29.pdf