I'm completely new to TCGs so i read up a bit and 2 common TCG concepts helped me out toward winning the majority of my games i play lately even with the default decks.
Card Advantage - Simply put was explained to me the total number of cards you have either in play or in your hand at a given time, keeping this number above your opponent's can greatly increase your chances of winning as you simply have more options and you're forcing them into a bad spot where they simply don't have as many strategic options as you do. Trying to coax out your opponent's more useful cards spell and taunt cards early in the game without expending your own cards can be of great use later on.
Tempo - Basically is picking your moment and controlling the escalation of the board. If you can take out some potentially dangerous cards using fewer cards than what your enemy played, you can turn a dangerous position into an advantageous one. Also, it's important to not "shoot your wad" all at once. Unless you're certain you can win the game in your next turn, you should avoid going all in because once you do, if your opponent counters, you're left with nothing.
Hope this helps.