Create an account in sandbox.paypal.com and create a buyer and seller account in it then do as follows:
1. Creating paypal form
<form method="post" action="https://www.sandbox.paypal.com/cgi-bin/webscr" target="paypal">
<input type="hidden" name="cmd" value="_cart"> <!-- if paypal cart -->
<input type="hidden" name="cmd" value="_xclick"> <!-- if paypal buy now -->
<input type="hidden" name="cmd" value="_xclick-subscriptions"> <!-- if paypal subscription -->
<input type="hidden" name="add" value="1">
<input type="hidden" name="business" value="seller_example@gmail.com"> <!-- seller email -->