How did the first 50 Bitcoins come into being? How was the Bitcoin system initialized? Can one start the system with only a single node (computer)? Please explain it in plain English. Thanks, guys.
The first block is known as the "genesis block" or "block 0".  It was mined using the same hashing algorithm as any other block, but at difficulty 1.  It contains only one transaction, the coinbase, which gives 50 BTC to address 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa.

Once the block had been successfully mined its hash was hardcoded into the Bitcoin client's source code so that it and it alone would be accepted as the official genesis block.

src/main.cpp contains this line:

uint256 hashGenesisBlock("0x000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f");