Learning to solo is a primary practice for guitarists. It takes time, so we start at the beginning. Our goal on this site is to teach students to improvise [make up melodies on the spot, in the now]. This is possible through the development of our melodic self, our melodic sensibilities [a lifetime quest]. We are also 'living in the middle', meaning we aren't playing a particular style, but the style that is created when we [you] experiment with the materials of music. The style is up to you. We can be surprised by what happens when we work through this basic lens. It can go any & everywhere.
Playing others' solos is an option, and we recommend learning at least one [Crazy Train is a good one], but we won't cover that on I Love Guitar. There are plenty of sites and resources available for you to explore playing existing songs and solos. Our goal is to begin & continually deepen our improvising/melodic self development.
For both of these parallel paths [learning existing solos/learning to improvise], we train. When we train beyond what it takes to play a particular solo, the solo falls into the hands with relative ease. And, the same for our own solos...we can only play within our existing limits [leaps happen however!]. With focused training, we expand that boundary. Our goal with improvising is to 'forget' our hands ['play dumb'], and just sing through the instrument.
When soloing, we have choices. Each of us chooses our approach [just playing by ear, using scale maps, thinking in chords/arp, etc.]. And, we choose our tone sets [scales, modes]. Playing over a chord progression comes down to knowing how tones work over certain chords [our ear also tells us this, whether we know what we are playing by name].
The typical saying is 'wrong note', but we are swapping note for tone. A tone is a sounding pitch. A note is a written symbol.
There isn't such a thing such as wrong tones, just wrong emphasis. In any given situation, every tone can work in some way. So, when we say 'wrong note', we really are saying 'wrong emphasis'. Some tones are passing tones [we pass through to get to emphasis or landing tone - emphasis/landing mean that we are targeting these tones on the beat or for longer durations]. Some are approach tones [typically a half step below an emphasis tone - we slide from an approach tone into an emphasis tone]. If we were to emphasize a passing or approach tone, it can sound 'wrong'.
The main tones which sound good all the time for a sounding chord are tones which are inside the chord [such as the root, 3rd, 5th, etc.]. Tones which are outside the chord can add color or tension [sometimes to the point of 'wrong' if not resolved]. Some need quick resolution [they are dissonant and want to go somewhere, now!]. Other tones, such as the 9, might not be in the chord, but it can add color, and sits well in the chord. The 9 is typically always a good emphasis tone.
We can't think our way into knowing all of this. We must experiment. And, for years.
A chord symbol is also a scale symbol. When we see a chord symbol, we interpret and realize it as a chord and a scale [and the plural - chords and scales]. Just as a chord can be voiced or expanded based on its symbol and context, the scale or scales that can be played over that chord is based upon what lives inside the chord.
We start by keeping it simple. We are simply experimenting with the sonic impressions that tones produce. Over time, our knowledge and experience leads us to use this information in ways that are congruent with who each of us are as soloers.
At the beginning, we can experiment with different tone/scales/mode by soloing over single chords [or a drone]. This gives us an opportunity to try any type of scale that fits the chord, immediately. We don't have to go the long way to figuring out more 'advanced' scales. They are accessible at the start.
This is super easy and not too common in Western music [very few one chord songs]. It is common in Eastern music by way of music being based on a drone.
The next easiest way to solo is over chord progressions where all of the chords are in the same key [by learning chord scales, we know what these are]. When we solo over a progression with all the chords in the same key, the modes [scale impressions] happen automatically [though we can approach each chord individually].
For this four chord progression, we can make one scale choice...C Major. The modes happen automatically. When the chord changes, the modes change without us having to mentally shift to a new or different tone set. What this means is that harmony ultimately determines how single tones sound within a sonic moment.
We can also approach each chord individually. Even though we can still use the modes found in C Major, we are thinking about each chord on their own. We don't have to use modes from C Major over the chords [go outside the tone set], but this idea of approaching each chord on its own [starting with modes in the same key] is how we move into more advanced soloing.
More advanced soloing includes progressions which contain a chord or chords which aren't in the same key. In this situation, it is essential to know what scale types fit with which chords. In this instance, the most advanced way to solo is to approach each and every chord in a progression as a single entity.
Above, since E Major isn't in the key of C, while Dm, F, & G are, we have to think about E Major differently. We can take each chord individually or just play in C Major, except for the E. A good choice for the E chord in this situation is E Spanish Gypsy.
These are the basic ideas for matching up scale types with chords.
There are many ways to think about all of this. The idea is to simply start. Start soloing, improvising. It takes time for our melodic self to emerge. S/he's in there, we just have to sing our way into the unfolding.