well, in the deck garage, they have tips to making a good deck, but I will assist you anyways.
1. First, you should think of what kind of deck you want. SP, or Evolution line. That is the first step, because they are very different decks otherwise.
2. Once you have chosen the kind of deck you want, think of the pokemon you want in it. For example, if you choose SP decks, you could use Luxray GL or Infernape E4, or Dialga or Palkia for example. If you choose evolution lines, you can use Flygon for example, or Gengar. Just examples though.
3. Once you have chosen the lines of pokemon, you must get draw power. Claydol and Uxie are the main draw power people use, and are difficult to obtain, as 99.99% of decks use them. Mainly the draw power consists of pokemon that have poke-powers, as both Claydol and Uxie both have them.
4. Now, you must choose your trainers. If you choose an SP deck, there are necessity trainers, like Energy Gain, Power Spray, and Poketurn. If you choose an evolution line for a deck, you should have Rare Candy, and Broken Time-Space trainer card. Now, if you run level x in your deck, you should always have premier ball. That is a card that searches your deck or discard for a level x. You can also run the less used, flippy level max. It allows you, if heads, to level up ANY of your pokemon, which includes benched pokemon. Every deck should use trainers like Luxury Ball, Roseanne's Research, Bebe's Search, the occasional Pokedex Handy910is, and Pokedrawer+.
5. So, now once your finished with the trainers and supporters and pokemon, you will of course need energy. There is of course basic energy, but some times you can other kinds of energy, like Multi energy, SP energy, and Call energy. Multi energy and SP energy can be any kind of energy, but only one at a time. Call energy ends your turn, but allows you to search your deck for 2 basic pokemon. This is very useful, to prevent what most people call a donk. Most decks run around 8-12 energy, which is usually all you need. Hope this little guide helps, but you should check the deck hints section in the deck garage.