There's a bunch of things you need to do/pay attention to.
I'll make a bunch of lists now.
Budget, because you need to pay for:
Musicians (concert and rehearsals)
Venue (for the concert itself and maybe if you need to rehearse there beforehand.)
Logistic (travel costs if people are not from where the concert is)
Misc (Food, lodging, whatever comes up)
Then venue:
Does the venue allow for rehearsals? How often, how long? Do you have to pay extra?
If you need a piano, does the venue have a piano (and if so, what kind, which tuning, etc.)
What's the lighting situation. Can you set the lighting yourself, is there a technician, etc?
Cost, of course. Sometimes venues will collect a percent of the ticket sales, you need to get that all in writing.
The meatbags with their noise-implements:
Certain instruments (usually stuff made of metal) need time before rehearsals to "warm up," take that into account when planning your rehearsals.
Dress code, which usually isn't a problem, but it can be with singers and, well, women. Not a "problem," exactly, but it's something to add to your checklist nonetheless.
Applause rehearsal, because seriously this is important and it reflects on you as a composer that everyone knows what the hell to do during applause.
Encore? Plan at least for two short pieces (or a repeat of a catchy bit from the concert program), because being optimistic helps group dynamic!
Keep rehearsals short and precise and try to schedule them either before or after lunch, since you don't want people hungry while they're trying to follow a 32th 7/16 marimba passage. If your rehearsal is so ungodly long that it overlaps with lunch (or dinner), then make sure you give your musicians options for taking a lunch break. This is your responsibility as organizer.
The actual concert, finally:
Organize your concert program so that if you suddenly need to move 5 marimbas and 2 pianos across the stage, you don't do it between short pieces. Keep large things on their own concert segments separated by a pause.
15 minutes is good enough for a pause, only do longer if you need to move 10 marimbas and 4 pianos, or something that requires very specific preparation (like tuning a harpsichord to a specific tuning, whatever.)
Calculate the time the concert will last. This is very important specially when you're presenting the project to interested venues. You need two numbers, the actual length of the music proper and the "real" length, including pauses between movements, stage setup, pauses and applause. Can also include introductions, and any other thing that happens during the concert. Round upwards always, things take much longer than they may at first appear.
Make a good and proper concert program that people can have during the concert to read up on important things like your biography, how great you are, and why everyone should be like you (great). Don't forget to ask the musicians if they are OK with including biographies for them in the program and allow THEM to give you their own text. Edit for size, but not for content. Also helps to add a list of the pieces performed along with their length and any other things you think are interesting to know. As a super helpful tip, if you can't fit it on a double-sided A4 paper, then you need to reedit.
And if all of this doesn't scare you, then congratulations! You're ready to annoy strangers with your weird pieces for dog whistle and garbage truck.