Reddit mentions of Games Workshop 99199999054 Artificer Layer Brush, X-Small

Good on you for jumping into it! First of all you definitely want to thin down your paints. Here is a good video on how to do so by the best 40k painter in the world. Secondly, if you are having trouble with some of the straight line bits like on the chest, neck, and leg pads of the commander, painters tape can help you practice and keep things straight. It's not the most reliable tool so I wouldn't use it as a crutch but it will help as you get more experience. Tamiya masking tape is decent and used a lot in the model painting community. Lastly, I'm not sure what size brush you're using but it could be that getting a smaller one will help. Keeping a medium brush around is good for painting large sections but I'd recommend picking up something like an artificer brush and painting with just the very tip of it to work on smaller detailed sections.