How do new nodes learn the topology of Lightning network?
This is done via the peer 2 peer network that comes along with lightning and is specified in BOLT 07 - routing-gossip. If you read the details you will see that you have to peer to a node in order to maintain a channel. But you could also establish peer2peer connections with other peers without the necessity to maintain a channel. 

The peer protocol supports channel announcement messages that are being forwarded via the gossip protocol and query messages to ask for messages if a node connects for the first time to the network or was offline as asked.