You are doing great! Your starting piece is already very even and looks fine.
Stoppers are nice but I only use them regularly on projects knit in the round on double pointed needles. That keeps things from wandering off the needles as I work.
My advice is to always finish the row so tippy areas only on one needle and then either stopper the needle or scrunch the project up toward the needle end and stick the pointy end into the yarn to hold it.