If you need info on anything you can buy with credits then you can click below :-
http://forums.x10hosting.com/rewards.php
If you ever need any information on your credit transers or if you simply want to check how much credits you have then you can click below :-
http://forums.x10hosting.com/credits.php
If you can remember those links then your good to go

By the way, to answer your question on the domain - you simply keep posting or completing other peoples others until you get loads of credits. If you ever need to purchase a domain with your credits then you'll need to save up until you have exactly 9000 credits which i think is quite cheap for a domain and would definitely be worth it! There is another chance you could get a domain and that is through the promotions. For this you just need to be posting regularly and you will automatically get entered into the weekly domain name competition - the chances are at random so there's a fair chance for everybody though i believe if you post about 5 posts everyday then you get an extra chance though i'm not sure so you'll have to double check on that. Don't forget to check the promotions forums for information on this. The twelfth weekly domain name competition is coming up to so you can only hope you win one
Hope this answers your question