Calculate the size of the edge tiles to plan your grid layout. If you want to have the edge tiles on opposite ends the same size you have two options using 24" tiles, and two options with using 48" tiles. The square footage of tiles needed and perimeter edging for the ceiling will be the same but it could change the amount of T bars needed.
Lot's of online places with excellent information on how to install a dropped ceiling. Not much to add to that. Draw out your grid layout. Plan where you place your light fixtures before hand. These fixtures need to be supported by tie wire or the grid, not by the tiles. Don't place the T bar support wires all over the place. You should only need them on the main T bar's and maybe a couple of other places where you need to hang something that has a bit of weight. And one of the most important items to remember; "Installing ceiling tiles with dirty hands is a must!!" because we all like little smudges and finger prints on our new ceiling, or not?!
Note: The edge tile sizes are based on grid layout not on what the actual tile size will be. The size of the tiles will be about 1/4" smaller than the results.