For What its Worth Starters Guide.

Currency/ Credits & Diamonds, now credits will come and go even when you run out there will be more eventually, Diamonds on the other hand are extremely valuable. You might be tempted to spend them on Credits, dont. You might want to spend them on Rare Crates, dont. You should only spend your Diamonds on Epic crates. For now you can not purchase Diamonds. As for Credits They are like water there will always be more so Max out your units as you see fit.

Playing as GDI/ Ok so in the beginning of the game you are severely underpowered and you may not get the same units as others in the beginning of your game due to EA RNG.

OPENING PLAY

This should be your opening play every time. Build a Havester then build a Barracks then build a Riflemen and a Missle Squad use them both to take two seperate points and from there you will be able to respond to either Vehicles or Infantry. Remember after you make 4 units the game slows the rate that you'll be able to pump out units. Its important to not try to swarm your opponent with cheap units because you will pumping them out at a delayed rate while your enemy will be able to make counters to your units and clear your occupied zones before you can hope to overwhelm him.

Alternate Opening Play

Build a Harvester then build a Barracks then build 2 Missle Squad send them after your opponents Harvester to get the Harvester Destruction bonus. This is a kind of all in strat you have to destory the enemy Harvester otherwise you've wasted a lot of resources for nothing. I suggested using Missle Squads because they are cheaper but you can use PitBulls but remember to build a War Factory instead of a Barracks.

Mid to Late Game Strats

Its hard to know what your battlefield will look like at this point but its important to get out your late game units like Titan or SandStorms as soon as you can. They will be used to block out your opponents units from taking your occupied zones. Always target your enemies weakest vehicles when taking a zone from an opponent.

Commanders

You will highly likely be using Lt. Strongarm unless you get a rare commander. The Minigun Turret is great because you can use it to block off unoccupied parts of your zones so that your opponent can not contest. You can use it to scissor your opponent between it and a attacking unit. You can use it to revel invisible units and generally save your arse.