To win intermediate you need at least 3 star. For advance 4.5 star. To get a lot of star you'll need a seed maker. This is how it's work:

1. Plant seeds like normal
2. Fertilize it
3. Harvest the fertilized crop ( you should get an increase of 0.5 star. More if you're lucky.)
4. Put the 1 star seed into the seed maker
5. Repeat step 1 with your 1.5 star seed

if you keep doing this you will eventually get a 5 star seed.

^just like master666z have said or if you don't have seeds maker yet, you can plant re-growed crops like tomato, broccoli. Then give it fertilizer everyday. I did it with broccoli and at the end of winter I can get 3.5 stars.

when you construct the makers shed, rebecca will start selling 4 different machines for the makers shed, buy the seed maker and build it for the makers shed, then put one of your crops in the seed maker, the seeds will have the same quality ass the crop you put in, repeat the process to have high quality crops :P

use fertilizer in the crops to raise ther level(use it once per week)

to win the begginers class you will need a star and a half crop
for the intermediate a 3 star crop
and for the advanced class a 4 and half star