First of all you need patience. If you are defensive you will win 99%.
We don't want to die...
Pvp - main stat is mass! if you have bigger mass in situation head to head you will always win.
1) So main strat is to buy top items that give you mass(size) + eat distance. You can outfarm all pro players + they need to attack you because they will lose if timer ends. This will make pressure on them and they will make mistakes with attacks.
2) Outfarming (defensive win) - you shoud remeber few simple rules.
1. Less circles - you woud eat more mass.
2. Don't turn to places you recently was.
3. Eat coins if you can (the risk if coin near virus or walls) also don't go for coins if there empty space without food it's don't worth.
4. Try not to use turbo, don't use it if you can, only needed to avade enemy attack (turning back for mass you used can be not worth it).
5. Try to eat food that is in piles but not in lines.
6. Small head turning can be worth due to eat distance, and no loosing travel distance.
7. Don't run near viruses you can accidently turn and tail will stuck in them.
3) You can push viruses on enemy if he is in good position to you and have big mass.(virus can took 100-300 mass if opponent have 1-2k mass) and you only use 30 mass points to throw virus. It's good tactic at slow farming games. Be carefull in pushing viruses, you can push it in bad angle, and it will not stuck in enemy.