This is a simple tutorial for setting who gets to start out with what buildings. Make sure Beginner mode under options is OFF.
First place the startpoints you want to use:

After this make the buildings/units you want to belong to you(this can be done as you please):

Once this is done open the trigger editor:

Create a new trigger and call it whatever:

Goto events, create a new event and set it to off at "Any Event" or #8:

Go under Actions and make a new action and set it to "Change House..." or #14. Set the player you want the building to switch to(Player @ A would be the player at start 0):

Now it is time to attach the trigger to the building/unit(s) you want to become yours, choose "Nothing" at the top of the white box to the left of your screen. After this you double click on a unit/building and a box should come up. Goto "Attached Triggers" and select your new trigger:

Repeat the final step for all objects you want that specific player to own.
Also, you have to create a new trigger for every player.