For those dates you'll need reservations to be sure of getting a seat. The simple rule is that if you are not sitting at a table then you will not be served. The only official exception to this is the Pig Pen in the Hofbräu tent. Don't ask, look it up. You can get served beer if you do not have a reservation but you'll need to be sitting at a table. Four guys are gonna have trouble getting space together.
With regards to what tent to go to, everyone has their own favourites for various reasons. Read any of the many threads already discussing this to see the various views that people hold.
If you're not looking to go where English is heavily spoken then I would suggest staying away from Oktoberfest. It's primarily tourists.
If you are going to try to make reservations then do it now or it'll be too late. You can find the numbers of all the tents
here. As far as I know, you must book a whole table which seats ten. The 'cost' of the reservation is the price of two beers and a half a chicken for which you will receive vouchers. I can't remember but it works out at something like €25 per head. Again, this has been discussed on countless other threads. Use the search function.
You do not pay when you get there. You pay when your reservation is confirmed as far as I know.
Good luck, and as I said, use the Search function. Pretty much any Oktoberfest question you want to ask has already been covered.
Oktoberfest threads - or at least the ones with "Oktoberfest" in the title.