I want to eat icecream.

Verb stem + たいです means “I want to ….”, so the above sentence in Japanese is アイスクリームが食（た）べたいです。

“verb stem” is the “-masu” form verb without “masu”, so you don’t need to worry about what kind of verbs (go-dan, ichi-dan, or irregular) you are using. It is one of the easiest sentence patterns to learn.

The only thing you need to remember here is that the subject of this sentence should always be “I” or “we”.

おとうさんはアイスクリームがたべたいです  does not sound like a sentence uttered by a mature person, although a father might say it to his own very young child(ren) who are still learning to speak.