Playing Live

As easy as it may sound, it is not just a case of turning up and playing!
The chances are that when you do your first gigs, funds will be stretched to put it mildly.  So you will not be appearing with all singing and dancing PA and sound equipment.  Depending on the venue there may be an in-house PA with a resident sound engineer.  However initially it is likely that you may have to provide your own.  And even if you do not, it can be beneficial to use an engineer that you know if you can - they will know you, your sound, what sound you want, etc and if they do your performances regularly everything is far more likely to run like clockwork.
Naturally, like the rest of us, sound engineers have to survive so it will not come for free unless it happens to be someone you know who will do it for a smaller amount.  One of your band members will hopefully be knowledgeable enough about PA and sound and this can save a fortune in the initial gigging stages.
It has been said before and i will again - every band should have a PA.  It makes financial sense and music sense.  It may seem a big cost initially but 8 or 10 gigs and it will pay for itself.

The Audience

A useful thing to have!  Initially you will be most likely heavily reliant on friends and family.  And try to get as many of them there as possible to start with.  It will keep the venue and promoter happy and make them far more likely to book you again.  Word of mouth is about the best way to get people to gigs - if people like you they will tell their friends and the audience numbers should grow each gig.  Don't just rely on the promoter or venue to get people in - do it yourself.  Until your name is at least a little known, you cannot expect too many people to pay to see you.
Naturally your friends and family will applaud everything you do.  Hopefully anyway.  But it is the venue clientele and others you don't know who you have to convince to come and see you again.  Even if you have huge amounts of original material, throw a couple of covers into your set.  It will get people paying more attention as it is something they know and they will be more likely to listen to the rest of the set - assuming the cover has been done well!  If the audience reacts to a certain type of song, play more of them.  It is all fairly obvious but the amount of bands who play their set with a "this is what we like and sod you if you don't" attitude is quite surprising.  Not everyone will like you and some people may leave but that is to be expected - don't get disheartened if you are playing in front of a handful of people and most importantly play exactly the same as you would if the room was packed.
Below average performance or a set littered with obvious mistakes is one sure way to empty a venue - and word of mouth unfortunately works in the negative too.
The audience needs to be engaged - you need to interact with them rather than just go through your set, song after song, with no interaction.  A charismatic frontman or frontwoman is generally the key to this - talk to the audience personally and they will respond.  And never give the impression that because you are on stage and they are paying to watch, somehow that makes you better - the whole rock star thing.  Unless of course you want the audience to not turn up again and think of you as something that is unprintable here. Your ego may well want building up but that is more likely to happen if you take the personal approach.
In the same way, don't leave the venue as soon as you finish playing or just talk to your friends - talk to the audience, listen to what they have to say.  Remember you are promoting your band and it is fairly difficult to promote if you don't talk to anyone!

Venue

To start with, it is almost a case of any venue is a good venue. However always try and go to a venue where the usual crowd is the type of audience you are after.  Go to venues, have a look and see what kind of crowd you get there.  Initially you can play for free just to get a bit of exposure - you can hopefullg make your money on CD sales, etc and it is something you will have to do, especially in areas where you are not known at all.
In any event always try to have a contract, however basic it may be.