Spend some time watching Stridah and Soter Dave tutorial vids on YouTube.
Get to understand Initiative, it can be a bit weird to understand at first, but it is key to understanding the flow of fights.
The concept of Gambling is making an attack when you don't have initiative, and gambling gets you punished when playing against better players most of the time.
Practice swing manipulation. This is the concept of turning your body during release phase of your attack to either speed up or slow down your hit. Speed up your hit by turning the same direction you are swinging, this is called "Accelling" your attack. You can accel overheads by looking down. Slowing down your hit by looking in the opposite direction of your swing and hitting during the end of your attack phase is called "dragging". This is a little more difficult to learn and id probably learn accels first.
Learning to Counter consistently will end up putting you above most of the pub lobbies, it gives you stamina back without having to land an attack.
Feinting is important. Mixing up which direction your attack is coming will help you pass your opponent's block.
Footwork, as well. Try and step around an opponent's block.
Game sense will come in time...the flow of the maps, where spawn waves come from, when to take a fight and when to disengage and etc.
Once you practice all of these concepts (the fundamentals, more or less) you'll be ranking top 10 or above in your Team Objective matches consistently and should very rarely have a negative KDR.
When you're ready to step your game up after that, start playing FFA and Duel servers. That's where the big boys hang out and you'll be humbled until you start to hang, but your skill will jump exponentially once you start playing those modes.
The way I practice is picking one of these things, and then just spend an entire game practicing it. You'll die more often until it clicks, but you start getting around level 60 or so and all the separate parts start to come together and you just start shitting on people more often than not all of a sudden lol.
Good luck, the game is a blast!!