Pick a subject that you are passionate about or an area which will assist you in your professional development. You are about to commit to leading a group of like minded individuals - you do not have to share motivation with your group but your motivation must drive you to devote your time to them.
Use venues that other groups use. They have done the hard work for you in finding the best places to meet in your area.
Avoid changing the date of the meetup. This is not popular and you will lose about 50% of attendance, just because people have full diaries.
Use meetup.com or FaceBook groups. These tools provide the group organiser with the tools you need and people know how to engage with it.
Turn up 15 minutes early to the meetup. You will need to be there to get set up and I guarantee someone will get there earlier than you.
At the end of each meetup, announce the date and venue of the next one, even if you have not arranged speakers. Diaries first, motivation to attend second. Your group will appreciate that.
Be prepared to be the centre of group comms, negative or positive.
People will see you as a leader - even if you do not see yourself as one.
Encourage shared ownership of content from the offset. Give and you will receive is very much the theme here.