While there are some true extroverts out there, who love walking into a room of strangers and effortlessly starting conversations, that does not resonate with most people. Thus, it's good to know that there are things you can do to make new situations with rooms full of strangers less intimidating.
1) Do your research.
If at all possible, find out who will be attending the event as well as a little bit about them. Take the time to visit attendees' LinkedIn or social media accounts, to find out more about what they do, their websites, and their backgrounds. If they are published, read what they have written. This will help you to plan your purpose (see next point) and may even provide opportunities for you to connect prior to the event.
2) Go with a plan/purpose.
Having a purpose (even if it is just to 'meet five people' or 'find someone who can give me advice on starting a garden') at a networking event makes it feel less ambiguous.
3) Set your arrival and departure times.
If you end up having an amazing time, you can always stay longer, but knowing your departure time helps lend to that purpose mentioned above, and provides more definition to your stay at the event. However, you should not be the first or the last to leave unless you have notified your host in advance of your other commitment. Finally, you should also rehearse a way to excuse yourself gracefully from a conversation so that you do not get stuck talking with one person/group the entire event.
4) Approach individuals who are standing alone.
Most likely, that person is feeling even more inhibited than you are, as he/she has been there longer, and your presence would be a welcome relief.
5) Practice your introduction and be the first to say hello.
While it may feel awkward to break into a conversation, the more you do so (extending a firm handshake and smile), the easier it will get and the more confident you will appear.
6) Ask appropriate questions and learn new things.
Steer clear of religion/politics/health conversations. Don't be afraid to plan out questions in advance. That way, if you do get stuck, you can draw on those.
7) Follow up.
Obtain business cards. Take advantage of social media. Mark your calendar to follow up within a couple of days, particularly if you said you would when you were interacting. Send something relevant to your conversation to demonstrate you were engaged. You may even consider connecting the person to someone else.