So now you have 4 people in a room, all with fancy instruments, gear and, in my case, a lot of effects pedals (but can you really ever have too many??) - all showing off what you can play individually But you want to play live gigs, so you need to play together, you need a set.
Choosing songs to play in a band...?? Well, there's a book in itself! Everyone has their own tastes and favourites of course - and it's great to hear new songs and artists - but what if someone doesn't like your bestest-ever 'This song is why Always wanted to be in a band since I was a kid' song ?? Even more complex when you try to carve out an identity for the band - you can't really be planing jazz one minute and death-metal the next ;) What works for your audience? oh wait - this is a new band - you have no audience! And sometimes there's also external input - friends, family, muses (everyone should have a 'Penny Lane' - see the movie ;) It's fun, it's frustrating, it's fantastic when you all agree on something AND you kill it on the first practice.
Now, you need at least 20 of these songs - 25 when your drummer plays so fast that your set is done too early:) They all need to sound great, please the audience, please the pub owner so you get asked back, please the band, and you need to be able to nail the performance. Practice, practice - every week, over and over. And the key skill, in any band, is learning to recover when you screw up - because you always will, when you least expect it too. Panic, lose your place and it's all over! But get it right and there's no feeling like it.
Most importantly - enjoy yourselves! I reckon the measure of a successful band is sometimes less about the quality of the music and more about the enjoyment of the band that then infects and feeds the crowd (of course performance counts too). I've see so many bands die on stage because they look bored or don't engage with the audience. Have fun, show it, and they will too.
personal tip: don't drink too much beforehand to 'have' fun' or loosen your nerves :) - you'll screw up, panic, lose your place and it all goes downhill from there.
Then... back into the studio - keep practicing your set. I used to find that a track un-rehearsed for more than 4-6 weeks would start to slip. But now you're getting bored of some tracks - start introducing new ones. Cycle back, debate, argue, acquiesce...
...and over time, you get better as a band, tighter, able to anticipate each others changes and it comes out in the performance.