Welcome to the world of competitive Pokemon! We will, as mentioned, not normally build decks for people but I will nevertheless be a little more specific that usual.
Starting with a box of Magma/Aqua makes it natural for you to make a deck based on these Pokemon. I have a preference for the Aqua Pokemon, but you may have to adapt to what you get in the box. Pray that you get a Blaziken EX or Swampert EX in your box, that can be traded for serious stuff (at least 3-4 regular foils).
The trick to the Aqua deck is to be able to use the special Aqua trainers, particularly Archie (you should get 4 in a box). Aim for a balance of about 20 Pokemon, 18 energy and 22 trainers. My deck would look something like this
3 Aqua's Electrike
3 Aqua's Manectric (at least one is mandatory, ideally 3 of the holo version)
3 Aqua's Carvanha (the one that can confuse)
3 Aqua's Sharpedo (preferable the holo version)
2 Aqua's Poochhyane
2 Aqua's Mighthyena
2 Aqua's Kyogre
2 Aqua's Walrein (can only come into play with Archie!)
I have aimed for a good selection of Aqua Pokemon with a good spread in weakness. You will probably have to make significant adjustments since it is unlikely that you will get (or even trade for) all this.
You would then use 4 Darkness energy and something like 9 Water and 5 Electric. You can easily trade for the darkness energy or perhaps get a Gym leader to give you some promos, you can get the other energy through trade or through the purchase of theme decks that comes with plenty of energy (I don't think there is one that specifically comes with this combination though). You will also get some energy if you participate in a prerelease tornament this coming weekend. Darkness energy is not expensive on ebay (the new M&A Pokemon will be!).
I will leave finding 22 suitable trainers up to you, just remember that Archie is mandatory in a deck like this.
This deck will probably not be as strong as BAR, Gardevoir and Ampharos (you can easily find deck listings for those), but it should be decent and not embarrass you in any way. Make another posting with what you can build when you get the box and we might be able to be more specific.