WhyIf you only have files (photos, music, spreadsheets etc) on your computer then you will ultimately loose them: hard drives have a fixed life span and one day it will fail: its not 'if', its 'when'. So you need to use an alternative place to store them. Previously we used to use 'local' backups: large disks at home that the computer will backup to, however, with the advent of smartphones and tablets it makes much more sense to use online services that are then available to all your devices.
Cloud StorageAll your files, e.g. letters written in Word or spreadsheets should all be kept in a secure online storage solution e.g. Google Drive (https://drive.google.com). In the same way as for the photos, there is an application to install on your computer to keep a folder in sync with this online storage:
In this way. when your computer dies, you still have all your files.
If you're neurotic, there are solutions like ownCloud that keep your files away from Google etc.
BackupNB: Cloud Storage solutions like GDrive or Dropbox are a syncing solutions NOT a backup, so when you delete the file on your computer its deleted online too. For backup, other cloud services like https://www.crashplan.com exist.
PasswordsPasswords should not be re-used: if one service is compromised (hacked) as we see in the press almost weekly, then its likely your email/password combination will be traded online with everyone else's, and then used to try defraud you e.g. steal your identity. Once this happens its very difficult/costly to 'go back'.
I recommend using a 'Password Safe' program in which to store your credentials. This is then a single password to enter that 'safe' and all your passwords are within. You can store this on a cloud storage (e.g. Google Drive see 'Cloud Storage') and access it from all your devices (phone, ipad, computer)
An alternative is to use an online service where you store all your passwords like http://www.lastpass.com/ The advantage is it's easier to setup and use, the disadvantage is that all your passwords are on their servers so you are trusting that they are keeping them securely, will stay in business and won't be hacked.
PhotosIf you're using mixed 'platforms' (e.g. Windows and Mac) then Apples iCloud won't work for you: it only really works within the Apple systems. Most alternatives offer some kind of application that you download to your computer which then uploads all your photos over time. You'll need to leave it on for a week or so for this to happen, depending on how many photos you have.
As most people have a gmail account you'll already have access to Google Photos and they offer an app to download here:
You'll likely need to tell it which folders to include/ignore and also to set it to 'upload original' not a reduced size version. There will likely be a cost implication (a few dollars a year) but the alternative is loosing your photos.