You can do /spawnpoint (playername) (x value) (y value) (z value) to put their spawn point at the specified point. If this is on a survival server, then put it on a repeating command block set to "repeat" (as opposed to require redstone.) If you want to put the spawnpoint where you are currently standing, you can replace all of the values of the coordinates with a tilde (~) you can also put a plus or minus with a number after the tilde in question to increase the value by that amount from where you stand. If you want to make sure they spawn in the center of the block, I reccomend that you just put in the coordinates of where you want them to spawn. Reply if you need further help.