I love thread painting. I wanted to learn too, so I watched you tube for hours. There are So many good tips, tricks and techniques there!
https://www.youtube.com/results?search_query=thread+painting+with+sewing+m achine
I found good advice right here to:
Thread Painting
The thread you use will depend on the effect you are trying to get, and the project you are working on.
When I first started, I used bottom line thread in the bobbin. DecoBob also works. I think any finer thread will work in the bobbin especially if you are doing a wall hanging.
I used an iron on stabilizer. Then I tried Terial Magic instead of a stabilizer. I have also done it with no stabilizer at all...just 2 or 3 layers of batting. All worked. So much depends on what you are painting or sketching. How close (dense) or loose your design is.
Different machines will want their own tension settings. You might just have to experiment with yours.
And using a topstitch or denim needle has worked for me. Something with a larger eye, and a sharp tip.
My suggestion is just play. Don't worry about doing it 'wrong' ... just use what you have, and have fun.
Remember to write down your discoveries so that you can repeat the ones you like.