Commandos can be an effective choice against all types of infantry, but due to their small regiment size (6 units) they will require extra micro versus sniper-based infantry. Likely to suffer some casualties against snipers, but at the low cost of $35 hope to win the battle through attrition warfare.
 
For commandos to be successful it's best to have an advantage in numbers, ideally 3 to 1 versus stronger regiments such as SaS. This can be done in the early game pretty easily, considering the cost of deploying 18 commandos is only $30 more than 6 SaS. However, as the game progresses, the regiment cap of 20 will prevent you from maintaining this advantage in numbers; so you will need to transition to mech or armor.
 

The basic build will vary some depending on map size and starting money (if more than $500)

If you are planning an early game Commando attack, or your base is located in the front, you might spend more on regiments and less on the HQ's (which bolster your economy). If you have plans on really getting your economy going, or your base is located in the back, you might spend more on HQ's and less on your armed forces. Of course this is an example of a multiplayer build (3v3, 4v4), if you attempt to eco this much in a 1v1 you would be quickly rolled...

Get a basic idea of which upgrades effect which infantry units, and how to apply those upgrades in online play. Included is a suggested upgrade order to help newer players understand which upgrades are the most benefitial to their cause. See: Upgrading Your Infantry/Airborne

There are two different types of formations, formation on the regiment level (Assault, Aggressive, and Column), and formations on the division level, which are a formation of a group of regiments.

Groups of regiments can also be put in the Assault, Aggressive, and Column formation by activating group behavior, but this is not recommended for combat, only for reorganizing your division if it becomes too spread out. That means most of the time your division should have custom formations with group behavior set to off. This also allows you to set the formation for each regiment in your division without affecting the group formation.

You will want to use different group formations for different game situations, some basic examples are seen on the left and right. The determining factors are usually what type of enemy regiments you are facing, and how many special op bombs your opponents have. Hotkeys are essential for effectively managing your divisions, so once you have selected and organized a group, save it by holding down [Ctrl] and pressing any of the keyboard number keys; or simply right click an spot on the group control bar.
 
Specifically for commandos, one method that is used often is rotating multiple divisions to keep the enemy constantly engaged. For example, take 20 commandos and split them into 4 divisions of 5 regiments each. Rotating two groups forward at a time will give you enough sniper units (10) to kill most infantry, and won't risk too much if your bombed (10 regiments $350). The ultimate goal of this tactic is to attack, and never stop attacking; cycling in new forces as soon as a portion of your forces retreat.