"I need an umbrella."
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.
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.