These are cards I would look at if you want to build a Houndoom deck:
Houndoom-EX
M Houndoom-EX
Volcanion-EX
Volcanion (attack Power Heater)
Houndoom is not super heavy on damage unless you are getting rid of energy, so having Volcanion with Power Heater will benefit you a lot, because it lets you get energy back from your discard pile. Volcanion-EX lets you discard a fire energy to do extra damage. So if you have 2 Volcanion-EX on the bench you are doing 80+30+30=140 (without getting rid of energy, if you get rid of energy you will be doing 160+30+30=220). Most Meta Game Pokemon will have between 180-220 health. And, if worse comes to worse, you can attack with Volcanion-EX.
You will definitely want to get Float Stone. Float Stone will make it so you can retreat your Pokemon for free. This will be huge if you have to attack with Volcanion-EX, since it can only attack every-other turn. Switch and Escape Rope will benefit you also, since it allows you to free-retreat your Pokemon.
As for "Stadium Wars", Parallel City is the most popular stadium right now. It knocks down either your bench or your opponents bench to 3 cards. This can be helpful to you either way, depending on if you want to get rid of cards you won't use as often. You will also want to get 4 Ultra Balls in order to make sure that you can get to your Pokemon - especially Hoopa-EX. And you will want to get as many Spirit Links for as many Houndoom as you plan on running in your deck. Personally, I would only do 2-2 line or 3-3 line max.
Hoopa-EX will also be a card you want to invest in. And if you aren't looking to buy Shaymin-EX right now, I suggest getting two Hoopa-EX. Hoopa-EX lets you get three EX Pokemon from your deck and put them into your hand. This will help with your build up of Houndoom.
Since you are going to be getting rid of a lot of energy, I also recommend getting a Super Rod. Super Rod lets you get 3 Pokemon OR Basic Energy (and you can mix that up) and put them back into your deck. Once the new set Evolutions comes out, you will want to get a card called Brock's Guts. It lets you get 6 Pokemon or Basic Energy and put them into your deck. Professors' Letter will be helpful, since it allows you to get two basic Energy from your deck and put them into your hand. I also recommend getting a card called Max Elixir, which allows you to look at the top 6 cards of your deck and, if you find an energy there, place it onto one of your basic Pokemon on the bench.
I know this is probably really confusing since you are just getting back into it, but if you need any more help just let me know! I can try to help you build somewhat of a deck list. You will want to be heavier than most on energy since you are getting rid of so much.