You refer to a unit by his name.
There is a tab in the unit dialogue which says name. You can call him anything, then whenever you are working with that unit you use his name. Any object placed on the map, including triggers, game logics and waypoints can be given a name.
When referring to the unit in the unit's own "Init" field, you can also refer to him by using "This". This is the correct way to refer to a unit in the init field.
Also, when it comes to naming soldiers, tanks and aeroplanes, you can give them an identity.
The identity of your unit is defined in the decription.ext. When you get comfortable with mission design you'll use this a lot.
An identity will give him a name, face voice and control some of these aspects further. For full details on the description.ext and identities see the wiki.
http://community.bistudio.com/wiki/ArmA:_Mission_Editing