You have two options to setup Dropbox i.e on your mobile devices or on your computer. Go to https://www.dropbox.com/ & sign in to your account. If you want to setup Dropbox on your mobile device such as Android, iOS, Windows, Blackberry, then go to the "Play Store" or "App Store" and search for the Dropbox and download the app. If you want to setup Dropbox on your computer, open your web browser and type www.dropbox.com and then sign in to your account.
When you download Dropbox app on your mobile device, it will ask you if you are 'Already a Dropbox User' or 'New to Dropbox'. If you are new user, then create your account by entering first name, last name, email and password. Once you have created your account, it will ask you to setup 'Camera Roll' feature. Camera Roll feature allows you to automatically backup photos and videos from your mobile device to your Dropbox account. Whenever you take a photo on your mobile device, it gets backed up to your Dropbox account automatically. It also ask you 'How often you want the backup to occur i.e. 'Only on Wi-Fi' or 'Wi-Fi + Cellular'.
Once you backed up photos to Dropbox, you can access them from anywhere on any device. You will see your uploaded photos in a folder 'Camera Uploads' located on your Dropbox. You can also delete your photos of your mobile device to free up storage space. You can organize your photos by putting them into separate folders. If you want to move photos from one folder to another, then just right-click on the photo and click 'Move' button and then choose the folder where you want move. You can click edit and then select multiple photos to move from one folder to another. You can easily share your photos with others if both of you are using Dropbox. To share photos using mobile device, tap on the photo and tap 'Share' button. You have the option to email, text message, Facebook message, Post to Facebook, Twitter or copy link to clipboard.