How do you improvise a countermelody? Listen to things in the music and respond: imitate, vary, fill in gaps. Which tracks, though? Start with music that is harmonically uncomplicated enough that you can predict where it’s going, but with enough rhythmic interest to give you something to react to. I do not recommend the blues for this purpose. It’s a popular strategy among well-meaning music educators, and I get why, but blues improvisation is not a beginner-level skill. So what should you use?
I like the first three and a half minutes of “God Make Me Funky” (or the shorter single version.) The tune is in E, and you can use Mixolydian, major pentatonic, minor pentatonic or any combination of the above. Try following and responding to the guitar.