Hey, I've been playing guitar for awhile now, but all I really play are covers. I would consider myself fairly decent and can play along to some harder stuff, but when people ask me to just grab a guitar and play, I cant play anything unless they have a specific song in mind. Are there any good techniques or other things I could do to where I can just play whatever whenever. Thanks.
learn the circle of fifths and all the scale boxes for whatever perticular style your into, just use the circle of 5ths like a guide to power chord crunching then solo over that. other than that just be original just because theres a preset scale doesent mean you can mess with it or even just randomly shred and figure out licks like that then when you have a few make a run
a thing that i've been doing recently is picking a key, and then learning all the notes on the guitar that work with that key, in minor/major scales. just play along to songs or chord progressions in your chosen key regularly, making sure you use the whole of the neck and try areas you didnt know worked and so on. go on a site such as http://www.all-guitar-chords.com/guitar_scales.php, and see which notes you can use.
When someone next tells you to pick up and play, you'll have the whole fretboard under your command! Also, create some stock licks and stuff to use with the new found notes on the neck in order to have something to fall back on if you fail to improvise completely. Hey, you can still say you were improvising.
Obviously go on to learn all the notes for other keys using this method until you can truly play in any key.
Good luck!
personally what i like to do (first you will need to learn your basic major and minor scales for this to work well) is turn on the radio and just randomly pick a station and start playing along with whatever comes on, not necessarily the song per se just improv a lead line over it, and even better is to try and fit something over something else that normally wouldn't fit (try something classical over something very jazzy, or a rippin shred over a mellow country tune) if you're just looking for improv'ing rhythm i would just recommend learning a little theory and some scales and messing around with them until you find some stuff that works that you like.
Well, to start out with improvising, I'd always advise beginning with the easiest to
get started with: Blues. Learn the pentatonic scale in all positions. That's pretty
much all you need to get going. That way you can concentrate on your phrasing
and making stuff up on the spot. Of course you can go quite deep with Blues and
there's much more to it than simply pent/blues scale, but you can sound good
with just that scale.

In my sig I did a quick, simple blues solo in A. It uses only notes from the 1st
position A minor pent box. Just an example of what can be done with only 5