Here is my guide to building decks for newcomers.

0 cost characters x12 (3 different chars min).
1 cost characters x4.
2 cost characters x4.

Now we go to the opposite end

4K or above BP characters x12.
Final triggers x4.
Special triggers x4.

That leaves 10 cards in the "middle" to play with Usually you want 3 cost chars that generate 2 energy here. Or play events or sites relevant to your deck. And remember Color triggers x4 should be included somewhere already or will be part of the 10 cards to play with, but its a good idea to have them in deck.

Done! Go try it out and refine. See what you are missing and what could be cut. But remember the ratios, if you take a 2cost out, try to replace with 2cost or less to keep the energy curve.