The first order of the day is to decide what assists suit you as a driver. It's very important when you begin to be comfortable with your level and what you want from the game, after all, the main order of the day is enjoyment! Here is a brief overview of what each assist does.
ABS: Or Anti-Lock Braking System. Set to ON, this allows the player to fully depress the trigger without fear of the brakes locking. It will however increase braking distances. Set to OFF, the player will have to modulate the pressure they apply to the trigger in order to find the sweet spot (the maximum braking pressure before locking allows for shorter stopping distances) and also allows the player to employ advanced braking techniques such as trail braking.
STM: Or Stability Management. This is designed to negate body roll and make the car corner flatter. This is the only assist I would recommend all new players to turn off immediately. I promise that you will never miss it.
TCS: Or Traction Control System. Set to ON, this allow the player to apply maximum throttle with minimum loss of traction to the rear wheels. The tradeoff here is that the TCS will also hamper corner exit speeds as it over compensates to keep the rear wheels in check. Set to OFF, the player will have to modulate the throttle, applying enough to get maximum drive but not too much which will result in loss of traction and in some cases, spins.
Gears: There are a few options here. Automatic, Manual and Manual W/Clutch. Automatic gears do exactly what they say but result in drastically reduced lap times. Manual gears allow the player to change gear themselves and Manual with Clutch allows the player to complete even faster shifts for maximum drive.
Braking Line: Set to ON, this will display a line around the track with rough braking and acceleration points. Set to braking only, the line is reduced to show where the game thinks you should lift and brake. It's important to note that these lines and points are in a lot of cases neither the optimal line or braking point but the line is a good thing to have in terms of braking reference, should you need it.
Steering This can be set to SIMULATION or NORMAL. Neither is a faster solution. Find what 'feels' best for you. Sim leans more towards oversteer but is more difficult to recover with, should you have contact or get into a slide.