This cannot be stressed enough. The venue for your event is the absolute most important item to get right and to get locked in.
An ideal venue should be addressing all of the following
Be able to accommodate the desired amount of people
Offer appropriate audio and visual equipment for the talks, such as microphones, projectors and power sources
Is there Wi-Fi available? If not, what is your backup plan (if any)
Be within budgetary constraints (see rule "Have you got an adequate budget")
Securing the venue as early as possible on the right date allows you to
Book in speakers – They can lock in the engagement
Announce the venue
Determine capacity – You now know how many attendees you can accommodate
Start promoting the event
Determine what approximate budget is needed