I have heard of Trees a lot but dont know what they are. And how do you get them gold?
Currently, you are level 6 on the French tree. That's pretty much it for that one. Tree is just the Duolingo term for the course structure.
As for getting it gold, as you progress through the lessons, they need to be reviewed occasionally. When all your skills are fully reviewed the icon for them turns gold. That's getting your tree gold. Many of us don't feel like we're keeping up with our skills unless all the old skills are kept gold.