[noun (place)] + に + [noun (inanimate)] + が + ある
[noun (person)] + は + [noun (inanimate)] + が + ある
[noun (event/occasion)] + が + ある
The verb ある is used to indicate the existence of inanimate objects, concepts, or plants. It can also mean "to have" when referring to inanimate things, ideas, or events. On the other hand, いる is used for animate objects.
The location (where something exists) is typically marked with に, and the existing object is marked with が.
ある is an う-verb.
It conjugates like a normal う-verb except for its negative form ない, which is irregular.
The polite 〜ます form is conjugated normally: