Here's a few hints:

If you have a shell or banana of any kind keep them. You will need them for red shells
coming towards you. When you know, throw the item behind you. It will miss sometimes.

When a blue shell or red shell comes and you are drifting, stop drifting and boost right
when it's barely touching you (you will need to respond FAST).

Do combos relating to their special items. Like the babies with their chain chomp, Bowser
and Bowser jr. with their massive shell. The koopas with their triple shells. Get a kart
with 3 star weight or more. NEVER GET A KART THAT HAS 1 STAR IN ANYTHING!

Train yourself. Do time trials until you get used to the course.

Watch videos on Youtube of people doing 150cc.

Look for shortcuts. It's extremely important!

Chill.

You don't have to win every race. I once had 30 points on flower cup 150cc and I won!

Yea, this was long. Well, follow these hints.

i played with a near expert but i was a beginner we had trouble so we played grand prix on
two player against each other but we agreed that he would win all of them and i would
attack on coming and use items. my incentive was that we were going to be able to win
stuff when we won.