I have been a MTG fan for a couple of years, and i want to get this, but before i do i need to know one thing, go you actually have to pay for the (digital) cards?

yep. its the same as normal magic. if u have a full set (like all of 8th edition, or all of onslaught) then u can let them be send to you as "real" cards.
because card values on magic online are different then in real, and often cheaper, some people make money in trading online sets in for cheap, then let them be send to them as real cards and then sell them on ebay.

That sucks, but how much do they cost per pack online? i am installing this right now

cost the same as real cards, since you can let wotc send you the paper cards when you have a complete virtual set.
this makes them worth about the same as paper cards.

How do you pay for the virtual cards? is it credit card or what?

if you have the real cards there is a card code input option i just dont know where the code is on the cards