The main aim of the game is to reduce your opponents clock to 0 while stopping yours from hitting Zero, to do that you need to use a mixture of the suits. If you're on decent health and the enemy is on low health playing Spades (double damage but no time restored to you) is your main attack, low on time? then use a Heart card to buy you more or a Diamond to give yourself a bonus shield if you know you're gonna take a hit next turn.
The different classes each also have their own abilities on a cooldown so Knight is probably a good beginner class as you get a bonus shield regularly.