"I need an umbrella."

[BJCUAl]

I assume you're asking for a translation. （私は）かさがいります. Unless you are looking for an answer using a specific word or phrase this is probably best.

[KeithWong9]

傘（かさ）が要（い）ります or 傘（かさ）が必要（ひつよう）です should be the most natural ones.

[BJCUAl]

I respectfully disagree about 必要. If the sentence said 'On a day like today, one needs an umbrella', then sure. Small difference, but nuance is important.

A「傘がいりますか？」Do you need an umbrella?

B「必要と思いますか」Do you think it's necessary?

A 「必要と思うよ。そとを見てください」You'll be in trouble without one.

B 「やっぱり、要りますね」You're right, I do need one.