Suomi

reason: name = なまえ、 need a topic particle は、need to slightly change how your name sounds to fit it into katakana sounds, and write です in hiragana so people don't think it's part of your name! : )

reason: 初める - first time 始める - to start something.

reason: 您好！我的名字叫瓦伦蒂娜。我会学习日语。我感谢大家！

reason: The 私は in the third sentence isn't necessary since it's understood you're talking about yourself. みんな would turn to 皆さん to be more polite. And よろしくね is also casual, so it can become よろしくおねがいします。 (よろしく is normally just hiragana, I think.)

reason:

reason: That last sentence of your English actually wasn't in your Japanese at all... Tried to add that in at the end. Keep the other changes the person before me suggested too!

reason: Changed the "don't speak much Japanese" since "much" wasn't in the Japanese at all. It only said "I don't speak Japanese." You also don't really need the 私の since it's understood you're talking about yourself. And I changed the "Sorry" to すみません because it seems more polite than ごめんね.

reason: Added a です after your name. There should be a が before 大好き. And I'm not exactly sure about the last sentence, so I used も to indicate also. Since it's supposed to be polite though, I'm not sure if you can just say アニメと漫画も。 so I repeated 大好きです.

reason: It's こんにちは[konnichiha]. And forgot です

reason: 今日 means Today.

reason: 書けます is the potential form of 書きます、 which would mean "can write."

reason: You could say "for 11 years" (using 間), or "since 11 years ago" (１１年前から). "11年から" by itself does not work.

reason: 話せます is the potential form of 話します, wich means "can talk"

reason: Kon'nichiha.