Don't die on round 1. There aren't any repercussions from dying on round 1, it's just humiliating.

Try to stack up guns or perks before round 11. Round 11 is hard. Big Baz will come for you. Try to stay in an area where he can't reach you.

Try to rush-buy the nuke ability if you are doing a melee-only build. The flying tentacle ♥♥♥♥♥♥♥ will start spawning every time you level up after round 3. They are near-impossible to kill without some kind of range.

Try not to stay in one spot for longer than 2-3 seconds. Else the flying mouths will start spawning en-masse.

If you manage to headshot the running big-headed dude with the sniper, keep the sniper and upgrade it. You'll thank me later...

Always use your movement to its full potential. Or at least try to. Use the slide as well as bunny hopping to always keep a high speed.
The longer you slide the higher you will jump right after, as long as you time the jump right. You can also nuke jump, because why the ♥♥♥♥ not.

Try n get used to the blood swipe mechanic to heal. Once you get the hang of it, it becomes second nature. Then you will have a real edge.

If you can get the most based weapon known to mankind (from the level up options) early, keep it. Its damage increases with every hit. Therefore it will scale hard and quick into the late game. It also has a lot of ammo.

If you get the flipping the bird emote, use it on every dead enemy body, on cooldown. You get money and xp multiplier points from each body explosion. It's also cool and it gives you the illusion that your life has purpose.

Best of luck.