And iI am 15 years old. And I am 15 years old.

Capital letter for "I"

I want theo learning English. I want to learn English.

You are mixing the tenses, "learning" and "to learn" are two different tenses and I am not sure what you meant. This is because you used "the" which is typically used before a noun not a verb.

For this, I watch thea series. For this, I watch a series.

There are many TV series on the Internet, since we do not know what series you are talking about then you should generalise and say you are watching a series. You need to include "I" since you are watching the series not anyone else.

My favourite series is Prison Break. My favourite series is Prison Break.

Spelling and grammar.

My friend said “You should watching the Supernatural.” And iI watched. My friend said “You should watch the Supernatural.” And I watched.

"Watching" is the present, your friend told you to "watch" since he is not saying to watch the Supernatural right now, he told you to do it in the past. Capital letter for "I"

OnThree word, iI love it. Three word, I love it.

Logic mistake, you said 3 words. Capital letter for "I"

And iI love Supernatural. And I love Supernatural.

Capital letter for "I".