You can schedule it to run immediately by choosing Run Once > Run Now and as long as you've approved the email before you hit Run, it won't have any issues.
Depending on your audience size, the email should start sending immediately although it can take some time for large lists to receive the email. Personally, I prefer not to go live immediately after set up, I prefer to schedule my emails the day before (at least), but it's physically possible to send out right away.
It sounds like you are describing a setup associated with the Email (mailbox) program. If so, this program requires you schedule it 15 minutes from the current time, and also includes a Head Start feature for larger and/or more complex sends which allows you to start program processing 12 hours in advance of the scheduled send time. If you need an email to go out right away, do as Christina recommends; use a smart campaign.