The forums is a great place to start. Ask around to find if any other bronies exist in your area, and ask if they're up for a meet up. /mlp/ on both 4chan and Reddit are other great places to 'advertise'. If there's a lot of interest, you may want to assemble a small team of people you can trust to make sure it all runs smoothly.
Websites are useful. Consider making one, especially if you plan on turning this into an event that runs on a regular basis. Include a Callander of planned events, even allow members to create their own if enough interest is sparked.
Make the event something people will want to attend, and consider pricing, some people may not have much money to spend. Obviously, you need to set a time and place, making sure that the activities you plan on doing can hold the amount of people that have RSPV'd. Also, it would probably be a good idea to collect contact information, so that you can notify previous attendees about future meet ups.
Finally, check out some sites and events planned by other meet up groups in different areas, get some inspiration, once you've done it once, it only gets easier.
Good luck!