To stake your coins, you simply just need a balance greater than 0. The more weight you have in your balance, the more likely you will generate blocks with PoS.
Yes, you could run the server on a computer always on to constantly be working for the network.
There is no masternodes.
To get an idea about how amounts to reserve/stake (gh2 posted recently about this):
Forging rewards will initially be very generous, expected to be 7-8% per month or more than a doubling of your HEAT balance in a year. To forge a block per day, you need approximately 3800 HEAT if about 50% of all HEAT is forging (26,500,000 / 3,456 * 50%). To double-check that you have set up forging correctly, your account should appear in this report: https://heatbrowser.com/report.html after the first block has been forged.
You can also take a look at https://heatbrowser.com/report.html
You will see a list of addresses who are forging, the balance of HEAT they have, and the amount of blocks they have generated. This can give you an idea of income from forging a certain balance would estimate. (Things depend on how much total HEAT is staking, like the quote above is considering 50% of all Heat staking. So it's hard to get an exact estimate until the network is running for a while and the amount of forgers is somewhat stable).
Rewards will be re-enabled soon. Then each block is rewarded with currently 8 HEAT (4 for PoS + 4 for PoP).
Currently the PoS and PoP is combined until it is needed to have PoP act seperately. That will be another layer of rewards which will work something like this:
For PoP you would have a computer running which will store blockchain data for the network. So you need hard-drive space to save files. In doing so, you will earn rewards too.