Where can I find an ATM?

Hold down Back (Xbox) / M (PC) / Select (PS3) / Touchpad (PS4), go to "Quick GPS" and select "ATM" which will put a waypoint on the nearest ATM.

This is from the GTA Online Guide - Interaction Menu:

Use the Interaction Menu to access the Quick GPS function, which will help you find your way to useful locations more quickly.
View the Interaction Menu in game by holding select/back.

Use Quick GPS to quickly navigate to locations such as your Home, nearby ATMs, Ammu-Nation and more.
You can also use Quick GPS while doing Missions – objective locations will appear as Quick GPS destinations.
Called Pegasus for one of your air or sea vehicles? Then select ‘Pegasus Vehicle’ as a Quick GPS location and go straight to your awaiting vehicle.

EDIT:

Here is a little more information about ATMs I found in the GTA Online Guide:

• You can also access your bank account to make deposits and withdrawals using the internet, simply visit www.maze-bank.com from your phone’s web browser or any connected device.
• Your bank is shared between your character slots so they can deposit and withdraw from the same account.
• GTA Dollars bought from the store will be deposited directly into your bank account.

The best way I've found to bank money is to use your phone, you can put money in the bank from any location.

1. Open your phone, go to the internet.
2. Select Money and Finances
3. Select Maze Bank
4. Deposit or Withdraw as much as you like

You can do this at ATM as well, but they are more dangerous, often being out in the open.