well to have a good deck, you want something that's either simple or complex, but something that is consistant, some ideas like t2 decks with only 1 pokemon can be consistant and so can setup decks, i honestly think setup decks have a huge advantage, in that if they don't lose t2 they will prob win the game,
as far as building one goes, you want a main attacker, a starter and a supporting pokemon for a setup deck
some examples of good attackers are
empoleon, infernape, electivire, blissey galade, magmorter, feraligatr, there are less conventional attackers but i think these are the best
some good starters are, pachirisu, lapras, mawhile, stantler, ching/chime, there are others but again i feel that these are the best
some good supporting pokemon are, delcatty, nidoqueen d, claydoll, porygon 2, minun, and lunasol i feel that these are the best
so basically what you wan to do is find an attacker you feel will have a good chance against other popular decks, not necessarily to try to counter them but to decide whether or not it will have a chance being played, then you chose the appropriate starters and supporting pokemon that go best with the main idea of your deck, try to develop some goals, like to energy draw to discard fires and then firestart them back on to pokemon, try to come up with the pokemon that go together in the way mag delc typh go together, they all work together to power magmorter, so come up with something that goes together like that, and works well against MOST decks and perfect it by adding in new cards that you found out through testing you would need, it's not easy and it will take a lot of time and practice, but everyone that is good and plays or comes up with a good deck needs lots of practice to get where they are, and for a lot of people that play at leauge they never get a chance to see the competitive sides of deck building, and show up at tournies unprepared for the amount of work their opponents have put into their ideas
everyone feel free to add or argue with what i've just said
hope it helps