The basic rules of Diplomacy are very simple:
-You have 1 unit per center that you control, which can be either army or fleet. Adjustments for gained/lost centers are made every other round. If you gained centers you may build new units in your home centers. If you lost centers, you need to disband units.
-All spaces can be occupied by a single unit only, otherwise a conflict occurs.
-All players secretly give orders to all their units, which are revealed at the same time.
-There are four possible orders
a)
hold - unit stays where it is
b)
move - move unit to adjacent space (with the obvious limitations that army cannot cross sea and fleet cannot cross land)
c)
support - assist another unit's move or hold order (important for conflicts). The limitation is that the supporting unit has to be able to reach the space the supported unit wants to move into (for move orders) or is in (for hold orders). The support order is cancelled if the unit is attacked by an enemy unit (exception: when the support is given to attack that enemy unit).
d)
convoy - fleet transports a unit over a sea space. If the convoying fleet is forced to retreat, the convoy is aborted.
-Conflicts are resolved such that the higher number of units wins, if tied the defender wins or the space stays empty when it was empty (this is called bounce). This resolution can be a little bit convoluted if a lot of units are involved. After the conflicts all defeated units may retreat to an adjacent space, except for the one where the attacker came from or where a bounce occurred. Any unit that does not retreat is disbanded.
The important thing is that you don't win Diplomacy by being the most strategic unit mover. You win Diplomacy by making other players help you with your plans. If you struggle with the rules in the beginning there are three ways to help you i) you can talk to another player if you trust him enough ii) you can contact the moderator (that would be me

) iii) there are programs
like jDip that can simulate the movement, so that you can test what works.
PS: Also what Walrus said
