This page will list some basic fundamentals of Marine Arena.
You have one Command Center, when it dies, you lose the game.
The main way to gather minerals is to kill enemy units. Every 10 minutes extra minerals are awarded to every player.
Your Command Center actually spawns a steady stream of Marines, at no cost to you, that you can build up to fight against other people.
On your Command Center, there are various upgrades for your marines, and also the ability to start spawning a second type of unit, the Mercenaries. These can be found on the bottom row, second from the left, on your Command Center Abilities panel. Marauders are a good choice for new players.
Depending on the Mercenaries you spawn, a different type of secondary building will appear. This building will constantly spawn (at varying rates depending on the mercenary) units for your army. NOTE: You can only spawn one type of Mercenary at once, any new mercenary building you purchase will destroy any previous one you have built. Your new mercenary building also, in most cases, has some upgrades for your mercenaries, and can be very valuable.
Your Command Center can also upgrade the damage and armor of your mercenaries, and allow you to use Stim Pack, a very valuable tool for making your army more powerful for a short while (note that stim pack only affects marines and marauders/maraiders)
There are Xel'naga towers positioned around the map, and just like in the vanilla flavor of the game, these give extended vision to your units, and are VERY valuable for knowing where to place your units to best fight your enemy.
Remember, force your enemy into choke points so more of your units can be firing into one place while theirs try to move through.