depends how long you played for prior to stoping play
it also depends on how long you stopped for
it also depends on how much passion you have to get back into the game

my advice is

do a bit of zone on a easy track like syncopia to just get used to fast speeds

then do some SL

then do some TT

then if you want to make sure do a few SR

then get online to race...

you could skip everything and just go online but you may just get frustrated when you lose games you were easily winning before you quit.but its all really down to mentality and how you handle winning/losing i guess.