I have choose in the begin of the game to be a good thief
But i don't know so god how to make potions
Does someone have some tips for me?

Mostly its just experimenting to find all the properties. Mix the things you find the most of which each other to see if they produce things.
I found all the properties of Red, Blue and Purple Mountains Flowers pretty easy because they are everywhere and easy to collect and test.

You can eat the ingredient to find out its first property if you want, though there are 3 others to discover.