If you have the option of getting the 16GB pre-installed, do so. I say that because I am using a micro SD card and it's been a pain trying to get any apps switched to the card instead of the internal memory. When I say it's been a pain, I do mean that it's like my card isn't even installed and it is. It has been formatted and everything.
Go to BestBuy.com, select the phone and then go to your service provider's website to purchase the prepaid service. Honestly, this was the easiest part of the process for me. I purchased the phone and then inserted the SIMs card from my old phone into the new one. I went online to Verizon and followed the steps to switch my device.
If you don't already have a Verizon account, just go to the website, click 'Sign Up or "Enroll" - I can't remember which one it is - and follow the prompts. From there, you should be able to activate the phone by yourself online following the prompts. You just need to click the "Add New Device" button and, again, just follow the prompts. If you have any trouble, click the live chat function and a representative will walk you through the process.
I hope this helps. Good Luck!