Hello !
I bet this has been asked before ..

How to get a rainbow candy flower ? I’m kinda new and I’m lost in several wild life 😖😖
Do I need to do anything or it’s just luck ?

TIA

The rainbows are dropped by the violets.

As per the wildlife guide (check the violet animals):
"Every 10th violet attracted drops a Rainbow coloured Candyflower (but only if you have an empty Hand-Painted Pot ready in your Garden for it!)"

Regarding your new player status, I would advise to maybe leave the candies for a little later. They were meant as an extreme blue diamond sink for people that have completed all other challenges and they offer no reward when they are finished. 

Hi Lizz - above is the link for the forum guide to doing the candy critters.  It is a bit of a test and see kind of challenge.  The idea is that you have to try different shapes and colours until you find the exact combination that will attract violet coloured candies.  Violet candies will appear when you are attracting orange, green and indigo of one of the candies at the same time.  Once you are attracting violet candies, every 10th violet spotted in your garden will plant up a rainbow candy.
So for example, can see in your garden that you are attracting red chocobugs.  This means that the shape in the left position is the correct shape but the wrong colour.  You have also attracted at some point green chocobugs which means you had the correct shape and correct colour in the middle position.  Since you know two of the shapes for chocobugs, you can concentrate on trying to find what shape you need in the right position to give you indigo candies. 
It can be a long process - do make sure you take good notes of what shape and colour has attracted what critters!