Use is in the when something is inside a place or container: The keys are in the drawer.
Choose on for surfaces (on the table) and at for a point (at the door).
Good: The milk is in the fridge because it's inside. Wrong: The photo is in the wall. A photo hangs on the surface, so say on the wall.
Writing a sentence about where something is
If the thing sits inside something, say it's in the that thing: My phone is in the bag. You're telling someone the phone is contained within the bag, not left on a table next to it.
If you mean a room or enclosed area, in the fits too: She's in the kitchen. The kitchen surrounds her, so listeners picture her inside it, maybe at the counter or by the stove.
If the space is only partly closed, in the still works: The kids are in the garden. A garden isn't sealed off, but it's a bounded area, and in places them within it.
If you're naming a large place, let the name decide. Cities, countries, and regions take in with no article: She's in Chicago. A defined area takes in the: We had lunch in the park. You're within the park's area, not touching a surface of it.
If the thing rests on a surface, switch to on instead: The photo is on the wall. Saying in the wall would suggest it's embedded inside the wall itself, which isn't what you mean.
If you mean a single point or spot, try at: He's at the door. At treats the door as a location point, while in the doorway would put him inside its frame.
When your place is a building, not a container
Use at when you're naming the building as a point on your route or errand list, not as a space someone stands inside: She's at the bank works when you're telling a friend where to meet you, because the bank is just the destination marker.
Switch to in the when the inside matters: She's in the bank, cashing a check. Here you're saying she's within the enclosed space, and that detail is the point of your sentence.
Both prepositions fit a building, so let your purpose choose. If listeners only need the general spot, at the office is natural. If they need to picture a room inside, say in the office: He's in the office, not at lunch.
Watch the common mix-up with large public places. Context shifts the wording: at the park often just marks the meeting point, while in the park places someone within its area. Ask yourself whether the boundary of the place matters to what you're saying.
Quick contrast in one sentence pair: She's at the school tells a parent she's somewhere at the campus, maybe in the playground. She's in the school says she's inside the building itself, perhaps signing forms in the front office.