In the same way folks need to figure out which software to download when they join the Bitcoin network.
When you join the bitcoin blockchain, you need some trusted source to tell you the hash of the correct genesis block.
Also, if you want to follow a shorter fork chain like Ethereum Classic, you also need weak subjectivity to tell you the first block immediately after the fork, otherwise you might be tricked onto the longer malicious fork, Ethereum :P
No, you don’t. There’s nothing special about the Bitcoin genesis block, it’s a block like any other. Whether you follow a chain that builds on top of this block or some other block has no bearing on the security of the system. It contains no keys that get to decide anything later on.