General Tile Discussion

I'll be installing 3"x 6" (true dimensions: 2-15/16" x 5-15/16"), 3/8"-thick marble subway tile in my shower. I want to have 1/8" grout lines and I will be installing in a half-offset pattern. What are the best spacers to use for a novice DIYer? Also, I assume I won't need tile levelers. Is that correct? Finally, I plan to use a 1/4" x 1/4" square trowel. Any guidance, comments, or suggestions would be greatly appreciated.

Thanks!