I try to think of it as that room full of stuff I used to have distilled into a
tiny, portable magic tablet with pretty much everything I could want or use at my fingertips.
First thing is to decide what you want to do with it all.
Then get an iPad or iPhone, maybe a case and stand.
Then you need an interface to get your instrument into the system and
the processed audio out, preferably one that uses the digital port on the iDevice.
I use and like the Guitar Jack 2, but other folks use other interfaces with great results too.
I also have a G55 Relay wireless which is totally extra, and a Radial ProAV2
direct box to handle the output which is less of an extra, IMO.
That's it for hardware.
There's supposed to be some Bluetooth MIDI controller pedals available Real Soon Now.
Then start with a modelling app. JamUp, Amplitube, AmpKit+ apps are all liked, more or less...
If you want to record, there's a bunch of very powerful apps. Auria, Cubasis, Garage Band, and more
Once you start to get a grip on the basics there's really geeky things you can do
with an app called Audiobus which allows you to use multiple compatible audio apps together.
No PA might be an issue for gigging, some people just plug the iMess into a bass amp...