Y + N to save your progress.
TAB + SPACE inside a car to save that car as your personal car.
TAB + N to call to bring your personal car.
TAB + Y to pick up a body. (You can take them to a hospital for some extra cash)
TAB + G to leave your weapons in a specific place (only if you have a bag)
TAB + H to release the bag you are carrying with you (without the weapons, just the bag)
H when you have a hat on to take it off.
2 when you are inside a police vehicle to activate patrol missions.
In missions, you will rarely have weapons by default, so it is best to stock up on your arsenal before starting a mission.
The best way to get money to buy weapons is by taking bodies to the hospital or doing patrol missions.
Save your game (Y + N) before starting a mission to have your most recent weapons available during the mission.
If you have problems with a mission (crashes, bugs, difficulty, etc.) you can press O + P to automatically complete the mission.
Gunfights can get complicated if you're not well armed or aren't careful.
In the Launcher version, you can lower the Storyline difficulty by disabling some gameplay features.