1. Communicate. You must have a mic and be able to communicate with everyone effectively (not just your squad, but commander and other squads).
2. Before you can be an effective first-in-command, you need to learn how to be second-in-command.
Help your Squad Leader - call out targets / directions / battlefield situation / where to find ammo / medic.
SQL wants everyone in a vehicle? Help him get there by calling each squad member's attention to go.
If SQL gives you a task, be competent in carrying it out without hand holding.
3. Know your squad - everyone's name, what kit they have, do you have medics with you, AR, LAT, Breacher, do they need medic / ammo / support, do they have a mic, if they don't - can they at least listen to orders.
4. Know your battlefield - where is the enemy, what is the next flag, should you attack or defend (its good to defend sometimes to let other squads attack), where is nearest repair / ammo / vehicle / medic / FoB.
Call out any targets with directions where to find them, help medics find wounded by knowing your men's position, don't call TRANS / vehicles to dangerous spots.
5. Tell your squad early and often what the plan is - puts everyone on the same page, so they don't wander off or do something against your interests.
For example: "When we cap this flag, I will call TRANS heli to take us to Bravo. Make sure you have ammo before we leave." or "We're going to defend this flag until Squad 1 neutralizes next flag."