Every agency is reviewed before an account is opened, so the first step is a short conversation about how you work. Once you're approved you'll get a welcome email and can sign in.
The order that gets you working fastest:
Add your venues first: they're the anchor for everything else. Then your acts, either by inviting them (they get an email and set up their own profile) or by adding them yourself. Then your invoice settings, so billing works from the first gig rather than being retrofitted.
After that you can create gigs and start sending offers.
If you're moving from a spreadsheet, add the venues and acts you're actively working with rather than everything you've ever booked. It's much quicker, and the rest can follow.