The “ー” prolongs the “i”. It's an artifact from katakana, a separate Japanese alphabet.
As for me, [I am] eighteen. "eithi-n" sounds familiar? It's the English "eighteen" adapted to Japanese. You find these kind of things very often in anime. However, many common Japanese words are directly adapted from English (and other languages) into Japanese. These words are always written in Katakana. "ha" (always spelled in Hiragana: は) is the "topic particle" and is always pronounced "wa". It's attached to a noun and identifies it as the topic of what you're talking about or as topic of the sentence. It's very important to keep in mind that "watashi ha" is almost certainly omitted, if it's clear that it is YOU who's being talked about.
More from Tae Kim's Japanese Grammar
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