How to ping someone on IRC

The person probably meant that you should start a conversation with someone. You need to know the network (or at least one server of the network) and someone's nick name to do that. Once you have connected to the server (network) where someone has been sighted, you can send them a message with:

/msg nickname hello, I was told to ping you. 

Not everyone enjoys private conversations on IRC, so it's considered good etiquette to ask the person if it's ok to send them a PM (personal message). You would have to know at least one channel where the person hangs out to do that.

Apart from that you could always /whois nickname to get their host and ping that, although given the context I doubt that's what they meant.

Well, there's multiple definitions of ping.

/ping <username> will send a 'ping' to a user (or more precisely a ctcp ping command), which while used for testing latency, is one way to let a user know something is up.

More informally, many users also go ping <username> in order to trigger off nickname highlighting (which is set in many clients) to cause the client to note that the nickname has been mentioned

It is also used in the same context as "drop me a line" or "call me maybe?"