I'm assuming you play 1.8. If you don't, give it a try, as it is nowhere near as difficult to learn.
First of all pvp. I will go through some tactics.
1. Sprint reset. The first hit of every sprint knocks them really far back, so if you reset your sprint after each hit, they will be knocked further back. The best reset imo is block hitting (press right click very briefly after each hit, only works with a sword), but you could also try w tapping, shift tapping or s tapping. This is necessary to pull off any kinds of combos.
2. Rodding. I had struggle learning this and still am by no means the best, but before you try to hit someone with a weapon, hit them with a rod (or any projectile). This stops their sprint, meaning they can reach slightly less far, and also their attacks won't knock you back far. If you're lucky, it will also knock them into the air, meaning they may not even be able to hit you. This helps get you combos very reliably when you manage to pull it off correctly.
3. Strafing. To make it harder for your opponent to rod you and even hit you, use your A and D keys to move left and right. Usually a zigzag pattern is enough.
Now, I will go over some skywars specific tips.
1. Try to get to mid. This is my main tactic, as there are very powerful items there, even more so in OP items. If you need to kill anyone on the way, then do.
2. Don't pick fights with people who have more gear unless they are on low health. It's common sense really. If they are the only person left, trying to hit them into the void or surprise attacks may be the only way of killing them if they are much more geared.
3. Don't chase people, or try to get to people across islands. It's not that smart really, they will bow you off or hit you off when you get close. Chasing gives opportunities for people to see you and come to clean. Let them come to you.
4. Take most stuff. Things like lava, xp and lapis, enderpearls, all projectiles and even just blocks can be incredibly useful.
If you would like to party me, please do! My ign is TechnoBoop, you can join in 1.8 and just party me and I will try to help.