What wall prep mistakes and grout choices should I consider for a backsplash?

[SofiaMM]

I'm tackling my first major DIY home improvement project by trying to install a tile backsplash in my kitchen, and I'm already second-guessing my preparation. I've watched countless tutorials on laying the tile pattern and applying the thinset, but I'm less confident about properly preparing the wall surface and choosing the right grout for a high-moisture area. For seasoned DIYers, what are the most common mistakes beginners make with tiling that I should be hyper-aware of to avoid a costly or ugly mess?