Scales to Play Over Em
Every chord is a subset of several scales: any scale that contains all of a chord's tones fits over it. Pick a chord to see the matching scales from its own root, the major keys the chord belongs to, and full fretboard diagrams for every scale on guitar.
A Em chord contains the notes E – G – B. Any scale that includes every one of those 3 tones fits over it — from the root E, that means E Natural Minor, E Melodic Minor, E Harmonic Minor, E Minor Pentatonic, E Blues, E Rock 'n' Roll, E Dorian, E Phrygian, E Dorian Bebop, E Half Whole Diminished, E Gypsy Major and E Gypsy Minor. Start with the first scale listed: it is the most common choice, and the others color the same chord tones differently.
Em is also diatonic to 3 major keys: C major (relative minor Am), D major (relative minor Bm), G major (relative minor Em). In a song from one of those keys you can also solo with the key's own major scale or its relative minor pentatonic across the whole progression, not just this chord.