There's several threads in the motorsport section that cover this.
I normally take:
A good attitude, be there to have SAFE fun.
Food, track food is expensive and often tasteless
Water, you will sweat if you are trying your hardest
Pen and paper to record your best times
Money for Wakefield licence, photo's, food etc
Your drivers licence
Clothing:
Long sleeve shirt, prevents sunburn and is needed for legalities
Helmet
Enclosed shoes, runners
Enclosed pants, jeans work fine.
Sunnies, turn that last corner and get blinded by the sun, not so good.
Your car, with new oil and car checked all over by yourself or mechanic recently (no good turning up paying money and your car gets knocked back for unsafe car)
Engine Oil, RB's use oil around the track and turbo's like oil.
Tyre pressure kit, at least a gauge, a pump if possible.
Tyre changing gear