Today, I've just had a walked with a friend and then helped my brother cutting his hair, but i. In the afternoon I havdidn't done much :(. Today, I just walked with a friend and then helped my brother cut his hair. In the afternoon I didn't do much :(.

We don't typically use "walk" as a noun. It sounds more natural as a past tense verb.

Maybe I'll try to have a productive evening, practicing with the guitar, doing stretching, and studying for tomorrow's tutoring. Maybe I'll try to have a productive evening practicing the guitar, stretching, and studying for tomorrow's tutoring.

With what you said, it sounds like the guitar is practicing something with you. You could also say "practicing playing the guitar".

I want to change this lazy period,. I should start again to gogoing to the gym again and wakeing up earlier so I'll have more time to usespend on my hobbies. I want to change this lazy period. I should start going to the gym again and waking up earlier so I'll have more time to spend on my hobbies.