Sherwin Williams, th paint department at Home Depot, Ace, or Lowes can all analyze a dime to quarter sized chip of paint.

If you have a can of the original in a closet or the garage or something, that's even better because it will tell you the brand of paint used. See if there's a local place that sells that brand). Every paint company makes really similar finishes, but they can vary just enough to be able to see a re-painted splotch in the wall. If you do the whole wall floor to ceiling, corner to corner, the difference between the same finish from two different companies won't be noticeable.

I'd recommend using a roller, and learning how to cut-in (great videos on youtube for making it easy), and use latex paint because you can clean up stray brush marks with a damp rag right there on the spot.