I usually play junimo kart via the web version
https://thomaswp.github.io/JunimoKartWeb/Source/index.html?mode=2&level=0
It has a bit more coins than the in game version, but it is easier to play for a couple of minutes and quit again.
As for the first level, I would say that the main thing to learn it how far you jump depending on how long you press the jump button.
The fruits do different stuff in the modes. In progress mode collecting all three fruits will give an extra life. In endless mode collecting all three fruits gives 5000 extra points. If you want to get 50k points in endless mode, getting the three fruits on the first two levels really helps.