First off, you need 60 cards. And you need at least one Pokemon in your deck. These are the only two things that all decks are required to have. Although in tournament play you can only have cards between the sets of Black&White and Boundaries Crossed. As such, it might be a good idea to keep your decks to those sets.
You need to decide what type of deck you want. This can start with a theme, but usually this will start with a certain card. I will take you through the build process of one of my decks, Volt Switch: http://pokemondb.net/pokebase/rmt/14125/volt-switch-electric-water-tcg-deck
What I started with was the wish to incorporate Black Kyurem EX into a deck. As it's energy requirements were Water and Electric, my deck's type was already defined. Usually you will want only one or two types in your deck. Any more and you start to run into trouble with your energy. Dragons and Colorless Pokemon can be incorporated into most decks though, and provide help by having different weaknesses and resistances from the other Pokemon, while also using the energy already used by the other types in the deck.
After deciding I wanted to make a deck around Black Kyurem, I looked through the sets to see what could help it. I saw its main weakness to be its inability to attack after using Freeze Shock and a hefty retreat cost of three.I saw the card Vanilluxe, which was able to give me a free switch of my Pokemon. The fact that it was a Water type made it a sure fit into my deck. It's ability also added another layer to my deck, playing mind games with my foe through use of Vanilluxe's ability combined with Catcher to make sure my foe is left completely helpless. You want to run a Stage 2 line as determined by how it will best fit your deck. A 3-2-1 line if you have trouble protecting your basics or a 2-2-2 line if you don't have that problem (or a 4-3-2/3-3-3 if you want to use a nine card Stage 2 line). Stage 1 lines can go as 2-1, 2-2, 3-2, 3-3, 4-3, and sometimes even 4-4. It all depends on how your deck works and how much space you have in your deck.
Next step was the rest of my Pokemon. Since I already had a Stage 2 line, I decided to strengthen my basics. You don't need a Stage 2 line in your deck, you don't need any evolutions in fact. But their abilities and power if you set them up are very helpful. If you do want to include a Stage 2 line, include only one, two at the most. It becomes difficult to run multiple Stage 2 lines, and three should not be done. I searched for basics that would be good for the deck and I managed to find the two Pokemon that make up Black Kyurem, Kyurem and Zekrom! Kyurem fit well as it's bench damaging would damage most foes enough to be brought down by Black Kyurem. And Zekrom was just a heavy-hitter. Since I had a basic heavy deck, I also added Emolga, which could easily bring out my basics while also attacking for little energy.
Now onto Trainers. This deck isn't as Trainer reliant as some of my other decks, such as Iron Maiden. In this case, Trainers are just there to support. I've got drawing, healing, and defensive cards. Choose trainers that fit well with your Pokemon and when you run out of those, just throw in staples such as Catcher and Cheren. Usually I have the number of trainers in my deck just be how much room I have left after I have my Pokemon and my energy
Energy is a thing that most new people have trouble with in TCG. I suggest running between 12 and 16 basic energy (you can add more Special Energy, as these are helpful beyond basic energies). It depends on the energy requirements of your deck. 14 felt good for this deck, so I went with an even 7/7 split.
Well, hopefully this helps you. And if need be, just post your deck and either Rio or I will answer it.
