はじめまして、私は(the particle "wa" is written は)サンダー(this is a closer transliteration in my opinion)です。17歳です (sai, written either as 歳 or 才, is used for age)。この方(this person)はティムさんです。
はじめまして、友達ですか。 (I prefer to avoid using "you" in Japanese as much as possible, and I don't think it's necessary in this case. Also, the question mark is not necessary, as the か particle denotes a question)
I'm not sure what you are saying in the third part. "yes we're friends. and you?" as in, you're asking for their introduction?
私はウチハサスケです。東京(とうきょう)に住んでいます。ティムさん(introduced as san, chances are you would continue to use that)は何歳ですか(何歳 or いくつですか are used to ask how old someone is)。
18歳です。 (since he's responding to a question and it's obvious that he's talking about himself, 私は can be omitted.)