Practice makes perfect. If you're at a networking event then everyone there is probably there to network. I think the hardest part is starting the conversation, but as soon as you start the conversation, it usually gets a lot easier.

I find it easiest to begin conversations near food or drinks because most of the time I'll pause, scout the room, and see whether I know anyone or how I have no one to talk to, and usually there will be another person doing the exact same thing. So it becomes very easy to open with one questions from the following like: Hey what do you think of the event so far? Are you excited for the event? What do you do? How do you like the food? How'd you hear about this event, etc. Pick one and see where it goes from there. You'll be able to tell whether they're interested in continuing the conversation or not, if they're not, maybe they're super hungry and they don't want to talk, or they actually need to talk to someone else. Don't take it personally but recognize social cues of when people don't want to talk to you. You can also tell when people are looking for someone or someone to talk to by how they're not talking to anyone and looking around the room. Opening a conversation with someone like that will usually be appreciated because most of the time you'll be saving them from that super awkward feeling when you know no one at a networking event.