ある and いる both mean “to be”, “to exist” but the usages are different.

Basically いる is for animate things like people and animals, and ある is for inanimate and/or abstract things.

• 街には人が沢山いた -> There were many people in the town
• 私は兄弟が三人いる -> I have three siblings
• うちには猫が二匹いる -> There are two cats in my place. I have two cats
• この辺には飲み屋が沢山ある -> There are many pubs around here
• 膝に痛みがある -> There is, I have, pain in my knee
• 今日の午後雨が降る可能性がある -> There is possibility that it rains this afternoon

However, ある can be used for animate things when they are an abstract group

• 仕事に多数の志願者があった -> There were many applicants for the job

ある can be used also for someone to be in some abstract position/state. This is a literary usage.

• 彼は四十年間社長の座にあった -> He had been in the position of the CEO for 40 years

The negations for ある and いる are ない and いない, respectively.

• 部屋には誰もいない -> There is nobody in the room
• この辺には飲み屋はない -> There are no pubs around here

ある and いる have different meanings when used as 補助動詞. Refer to this link for the details.

Also note ある can be written either 有る or 在る. 有る is usually used to mean both animate and inanimate things to “exist”, while 在る is used for an animate things to physically be at a certain location, even though these are not definitive rules.