Hello Valentin! Welcome to the forum!
Great to hear that you are motivated to improve with us!
Regarding your first exercise: what helps me a lot when drawing straight lines is to rotate the canvas in photoshop to get an angle where you are most comfortable! Drawing straight down is tough so maybe give it a try and see if you can straighten out these lines.
Also: I dont know how fast you did those, but maybe try to do them a little faster and with more confidence (dont focus on the line hitting the end point, focus on a straight confident stroke).
If you want more info on those very basic exercises and techniques, you can visit drawabox.com. They have very good explanations and exercises to practise lines, ellipses, circles and boxes (as the name might indicate )
I wish you the best on your journey to becoming a better artist.
Cheers,
Mau