First of all, you have to differentiate between a concert situation and a bar/restaurant. What I'm about to say may sound just a bit cynical but that comes from 40 years of dealing with the people who make the decisions about music. You may luck out and find bar owners or concert promoters who treat you with respect but unfortunately that is less common than it should be. But let's put that issue aside for the moment and talk about the logistics.
Always remember that a concert is all about the music; in a bar or restaurant the music is secondary to the reason most people are in the place in the first place: the food or drink or both. Because of this fact you should always assume a couple things. A concert may or may not include a P.A. system and a person who knows how to run it. A bar situation usually does not so you have to provide your own sound reinforcement. A concert stage almost always has plenty of room to set up and plenty of electrical outlets; a bar manager may want you to set up in a very small space (space is all about tables, after all) and there may be limited electrical outlets close by - or none at all.
Because of these facts it is vital to prepare for just about anything, that is, bring the least amount of stage equipment you can to still be able to get the best sound but bring plenty of stuff that will ensure you can play at all. Firs of all, bring PLENTY of grounded, 3-prong extension cords, at least one power strip (two is better) and a couple of those old-school 2-to-3 prong converters - I've played more than a few places where they were necessary, which is surprising every time. In fact, one place I played a few years ago was very popular but quite old and all the outlets were the 2-prong type. After the fire that destroyed the place shortly after I was given a long term engagement there (damn!) the investigators determined the fire was caused by an insufficient and antiquated electrical system.
You may find a place has its own P.A. system, which sounds like an advantage in terms of schlepping equipment but that may not be the case. One place I know here in town has an awful system that the owner insists the performers use and every performer is challenged to get a decent sound from it. On the other hand, some places do have wonderful systems but they may not include monitors, so take that into consideration when packing your gear.
As far as that's concerned, if your intention is to play out on at least an occasional basis, the second most important piece of equipment you can buy besides your guitar is a good P.A. system. There are some terrific, small easily transported systems available these days but as with most things, you get what you pay for. The Bose L1 system is fantastic but be prepared to pay upwards of $1200 for one - USED!! Fender makes some neat systems that fold together so the speakers and amplifier/mixer are one unit to carry. I have yet to hear one that sounds great, although they are passable and can be had for about $500, including stands.
Re: stands. Always a good idea and in my opinion, just about a necessity with P.A. speakers, not only to disperse the sound without overwhelming volume but also to minimize the "footprint" of your gear. Having to use chairs to prop up your speakers not only looks unprofessional, it leads to muffled sound that is aimed low, but most importantly using those chairs means there are two or more chairs that can't be used by paying customers. This is NOT the way to endear yourself to a restaurant owner!
Even if a venue has a system I always bring my own, just in case. That way I know how to present the best possible sound. Whether or not you can use it is another question.
How about other things on stage? Microphones and mic stands are essentials of course. For decades the Shure SM58 and SM 57 have been the standard mics of performing musicians, even sometimes in very large venues. They sound good and are just about bullet proof. If you're buying new ones, go for the models with on/off switches (and be sure to use them if you want to comment to your bandmates about the young lady with the short skirt at the table in front!). Also, be sure to bring at least one spare mic cord. In fact bring a few spare cords for the guitars and amp to speaker connections. Be sure they are all at least 10 feet long.
Don't those guys on TV look cool with all those guitars lined up on stage? That ain't you. Assume you will have room for perhaps two guitars and of course bring stands for them. Leaning a guitar against an amp when you take a break is an absolute disaster waiting to happen.
Spare strings (a couple sets), spare capo and clean cloth to wipe down the strings on your guitar when you're done are also necessities. Some other small items to consider are foam mic covers if you're going to be playing outside (wind through a mic is very annoying), a roll of both electrical tape and duct tape, the latter for taping down and covering cords leading to speakers that may be tripped over by audience members - or band members! A Leatherman-type multi-tool has doA few large plastics bags are a good idea too if you're playing outside and there is any possibility of rain. In which case, by the way, you should IMMEDIATELY STOP PLAYING and turn off all gear, even if you're in the middle of a song. Shocks are not fun.
Finally, back to dealing with the people who make the decisions. Your best course of action is to be crystal clear about starting and stopping times, how much volume you should or shouldn't use, and when and how much you will be paid. That way there are no surprises.
One last point about logistics. Set up as soon as you can. Do not wait until 20 minutes before you're expected to play. That way you can deal with any little glitches that may and most likely will occur. Setting up early also means minimal disruptions of people sitting having food and drink. If you do need to set up while people are in the room, do whatever it takes to not intrude on them.
Then - have some fun up there! Remember that audiences will forgive almost anything if you look like you're having a good time and know that respect is a two-way street. Let the people know you appreciate them being there. Smile, for God's sake. It isn't hard!
Peace & good music,