At
Used to point out specific time:

I will meet you at 12 p.m.
The bus will stop here at 5:45 p.m.

Used to indicate a place:

There is a party at the club house.
There were hundreds of people at the park.
We saw a baseball game at the stadium.

Used to indicate an email address:

Please email me at abc@defg.com.

Used to indicate an activity:

He laughed at my acting.
I am good at drawing a portrait.
In
Used for unspecific times during a day, month, season, year:

She always reads newspapers in the morning.
In the summer, we have a rainy season for three weeks.
The new semester will start in March.

Used to indicate a location or place:

She looked me directly in the eyes.
I am currently staying in a hotel.
My hometown is Los Angeles, which is in California.

Used to indicate a shape, color, or size:

This painting is mostly in blue.
The students stood in a circle.
This jacket comes in four different sizes.

Used to express while doing something:

In preparing for the final report, we revised the tone three times.
A catch phrase needs to be impressive in marketing a product.

Used to indicate a belief, opinion, interest, or feeling:

I believe in the next life.
We are not interested in gambling.