Hey guys, I've managed to set up my first colony, on Mars, but have some questions.
1: I'm getting messages saying that 'Unrest is rising on Mars due to overcrowding'. What can I do to stop this, and what happens if I don't?
Put infrastructure on the planet. Currently your civilians are dying because there is nowhere for them to live. You can suppress the unrest by sending in ground troops. Ability to suppress unrest is determined by their defence rating so garrison units are the best in terms of ability per cost.
The longer term way of dealing with this is to terraform the planet to the point you don't need infrastructure. I use terraforming ships for this instead of terraforming facilities as they don't need population to operate.
2: I have dropped some mines onto the planet, but what else should I put there, both for benefit to myself and that the population will need?
Assuming it's not a colony cost 0 planet you will want to move infrastructure there, to allow population to live there without dying off. You can build infrastructure in the same way you build installations. I would generally go with using automated mines instead of normal as then you don't require population for their operation. Shift some construction facilities or engineer brigades over to the new colony so you can build there. Again engineer brigades don't require population to operate, but they take much longer to train than to construct a construction factory. Once you have the construction capability a tracking station is a good idea to expand your sensor net.
On another point, I'm trying to design an Asteroid Miner - how do they work? Do they need a large cargo hold for carrying the minerals, do they need cargo loaders? How big should they be? How many mining modules? Do they need sensors?
Asteroid miners don't need cargo holds. In order for them to mine you first need to create a 'colony' on the asteroid using the F9 window, select the asteroid and hit add colony. Give your asteroid miners a move to order to the new colony, once there they will automatically start mining. The minerals they mine are placed on the asteroid. Have a cargo freighter do a pick-up run once there are sufficient minerals to be worth it(2500 minerals per hold), or transport a mass driver and set it to send packets back to your main construction colony.
They don't need to be fast as they are going to be spending most of their time sitting in orbit. Remember to make them a commercial design so as to avoid suffering maintenance breakdowns. Number of modules is dependant on if your going for small ships or not as a general design, Normally I'd put 5 modules on a basic miner, as that puts it in the same size range as you should be designing your freighters with 5 cargo holds. They don't need sensors to do their job, but personally I give sensors to all my craft. Sensors remain commercial as long as the size is at most 1.