You either mine a full block...
-> Then you're free to take the full 12.5 BTC reward or less, at your choice.
-> You can mine with a pool. Everyone in that pool calculate hashes of trivial amounts, but their sum will be enough to mine blocks. Then, the pool will reward your work. (Search for PPS or PPLNS)
You can use the -prune command when you execute the core. This will strip down the blocks your client maintains to only the essential. For me it has pared down the size of the btc Blocks folder by a third.
Add -prune to the end of your command line when you go to run the bitcoincore executable
All command-line options (except for -conf) may be ...