Improvising is very much about listening and communicating with other musicians. To be able to "speak" the musical "language", you have to learn the letters, how to build words and sentences. In guitar terms: To learn the fretboard and know where all "letters" (tones) are. Then try and learn some of the most important basic words in the language: how to put the tones in orders that will make you able to build "sentences" (phrases and solos)

The first thing most electric guitar students learn is the major and minor pentatonic scales, which are both the easiest and most commonly used scales. The scales can be found in tab here on guitartricks.I won't get into so much theory as building these scales, so get a good book about music theory. Learn the two pentatonic scales in five different positions (you just have to learn five positions because if you play a minor pentatonic scale three frets up you get a major pentatonic! The theory books will explain such things better).

When you know the pentatonic scales then put on your favorite record and play along with that. Try to play your own solos using the notes in the scale. Try to copy some phrases and licks from the record (older blues guitarists are easy to learn from since the don't play too fast).

Then learn some other scales, for example the blues scale (it's a minor pentatonic scale to which you add an b5). Different scales are usable for different situations. There are jazzy scales, metal-sounding scales (ie harmonic minor) and so on..
So...the long-time goal...learn the fretboard so well that you can just play what you want for the moment. And for now: pay around with the scales you learn, and copy licks from other guitarists. And buy a good book explaining things better...

/Wide