I am new to magento, after I create my site, I want to integrate a payment gateway. My question are  

1) How do I integrate payment gateway and which payment gateway should I use?  

2) Can I use free version of magento for commercial purpose ?  

3) After integrating the payment gateway, when customer buys a product, does magento charge transaction fees ? (Important question)  

4) How much percentage does the payment gateway(paypal or ony other) charge when customer buys a product ?