Every unit is different, and some techniques are unit-specific so we won't be discussing them here.
One very common way of assigning call signs is by unit (e.g. Bravo Company) and then by subordinate unit designation.
So for example, a platoon leader of Second Platoon, Bravo Company might be Bravo-2.
Additionally, there are designations loosely based on the Napoleonic staff model. For example,a battalion personnel officer might go by the battalion nickname (e.g. Bulldogs) and the staff designation (S1). "Bulldog-1."
Under this system, the "6" generally represents the leader, "5" is the deputy or XO, "7" is the noncommissioned officer in charge.
"Actual" means the actual person assigned the call sign (e.g. "This is Bravo 2-6 actual," as compared to someone else answering the call on that person's behalf. The person carrying the radio often adds an R or "Romeo" to the call sign to limit confusion (This is Bravo 2-6 Romeo, over!).
Those are common ways of doing call signs. Some units use nicknames, some assignments are random. It depends largely on how much OPSEC is required. I've been in units where we just used peoples' names.