I've started studying English. I've started studying English.

I have started studying English. I have started studying English.

It's grammatically correct, but there's a subtle nuance. If you focus on the fact that you "started", leave it as is. (past simple) If you want to emphasize that you're going to continue, "I have started ..." (present perfect) Since this is Day One and you intend to continue, it's better to say "I have started ..." / "I've started ..."

My English skills are still very poorlevel is still elementary, but I want to change my life because life is about taking on challenges. My English level is still elementary, but I want to change my life because life is about taking on challenges.

"My English skills" sounds vague and awkward to me. If you name specific abilities, like "My English writing skills are poor", "My English speaking skills are weak", then using "skills" sounds appropriate.

So, I practice English sentences every day. So, I practice English sentences every day.

It's grammatically correct, but since your title is "Day One", it sounds unnatural. You can say "I'm starting to practice ..." or "I'm going to practice ..." or "I've decided to practice ...".