Okay Trach, I see what you did there.... I always start with Mewtwo... >.>
First off, hello fishcherconnet, welcome to the site! When it comes to tcg, myself or trachy are your main people that have a wide experience with many different aspects of the game. If you have any questions, one of us should be able to answer it.
What Pokemon cards to start a deck around... Well, if there's a particular card you happen to really like, you always start with that as a basic and build around that.
For example, I myself am very fond of Mewtwo EX, and usually try and incorporate it into my decks. Since Mewtwo's primary attack (X-Ball) deals damage based off of how many energies are attached to both Mewtwo EX and the opponent. So using Mewtwo, I like to use energy acell decks such as Rain Dance, Reshiboar, Gardevoir, etc.
Using cards you are familiar with, with strategy, and are very well aware how it can function in a variety of decks gives you a more successful win rate
Custom decks are a bit trickier. When building custom decks, you have to be aware of all the more standard decks. Decks like Klingklang/Coballion are popular right now, and are very successful when played correctly. Somewhat like Haymaker, Klingklang/Coballion corners off the daunting EXs of our time, and attacks with powerful basics. Once one figures out how the deck works and gets more comfortable with it, learns it's strengths and weaknesses, it is an amazing deck.
For the fact, one time, someone actually got a perfect set-up, using the time I couldn't attack with my EXs, my psychic weakness, and the general struggling I had. It's powerful, but awful common, so it's also useful to know how to take on the same deck you're playing when you're playing a more standard deck. It's good to be familiar with everything, it also lets you in on what your opponent may be planning, almost every deck has a few things it must set up before it an conquer.
I probably did a terrible job explaining this. Way to many technical terms. I apologize if this doesn't make much sense. I bolded the most important lessons. Good luck in your future battles!