Hi Jeni
The first thing to do is to pick a theme. You could make it a quiz night or a poker night. Then you need to find a venue, ideally free of charge - you could see if the local pub/bar will let you have it there.
You work out how many people can fit, then you do a budget and calculate a ticket price that will allow a good profit. Then you start marketing the event and ideally you want people to buy a table and get all their friends to come as this is easier than selling individual tickets. Produce posters, send emails, do everything you can to market it as a fun night out for everyone. You want people to want to come because it will be a great evening and not just because it is for charity.
I hope this helps. let me know if you need more advice.