Alright, I'm just going to tell you about spawning reinforcements. The rest that I wrote, if you want to make that then you can learn yourself, but I can tell you all that too if you want me to.
All such work is done using Scripts. In the toolbar, near to the right end you'll find a button named Edit Scripts. Click on it.
Click on it will open a window containing the folders for each player. Select whatever folder you wish (I advise the folder for the player in question). Then click on New Script. Another window will appear.
This is the script window. There are four sections: Script Properties, Script Conditions, Actions if True and Actions if False.
In the Script Properties you can adjust your script, but I advise leaving it the way it is (except the Script Name which you can change as desired). Then in the Script Conditions section, click on TRUE (an already present script condition), and then click Edit. You'll see a vast lost of Conditions to chose from. We'll get to this later.
In Actions if True section, select the already present action and click Edit. Again you'll see a vast list of actions. Go to Unit (includes Structures) -- Other -> Spawn. Here you'll find a list of spawning actions, choose whatever you desire. When you have chosen, in a field near the bottom your chosen action will be displayed, and you can edit the blue coloured parts as desired.Now, as for the Condition, you could either leave it to the default TRUE (so the script will fire as soon as you start the game). But it is more likely you want the reinforcements to arrive after a set period of time. To do this, you'll need to make another script:
In the new script, choose the Condition as TRUE (the default) and for the Action if True, select Scripting -> Timer -> Seconds countdown timer -- set. Here you can write a name for your timer, and the amount of time for the countdown timer.Next, in the previously created script (select it and double-click on it), change the default TRUE condition to Scripting -> Timer expired. Choose the timer you just created, and you're done!