Order wise, Well, the sheet is read from top to bottom (this matters when you are using Else, or when two events are conflicting), triggers can be put almost everywhere without any difference (unless particular case where you have twice the same trigger, which is again a conflict case), also, all the active event sheet is read before the game goes to the next frame, so do not worry too much about if ennemies or player is first (event 10 or event 500 will still happen unless you have something preventing it in between).
condition wise, they are checked from top to bottom, so try to have the less demanding condition first if it is nessecary (for collisions that do not benefit from the collision cells, it can help sometimes), as if one condition is false, the others wonb't be checked, however as spongehammer said, in most cases, just do what is the most convinient, groups being disabled can help performances wise, but organisation comes first anyway and they are great for that too.