Here is what you are gonna need:
-A high speed internet-cable, dsl, etc.
-The cost of xbox live ranges from like $7.99-$59.99(prices may not be exact but are probably close)
-You can buy a subscription for xbox live when you connect. You can just pay with a credit card.
-If you want to download any thing you are going to need Microsoft Points.(some things are free to download but most arnt free)
-Your prices for Microsoft points range from $6.25-$59.99 or higher depending on how many you buy. I just recently bought 500 Microsoft points for $6.25
*All prices are in US dollars.
*If you have any more questions I will be happy to help you.