To follow a profile, visit it and click Follow. The button changes to Following. If that profile approves its followers, your request shows as pending until they do, and you're notified then.
New posts from the profiles you follow appear in your timeline. Profiles You Follow, in your user menu, lists them.
To stop following, click Following on the profile, and confirm.
The Timeline on your profile shows what you do on the site: new items, profiles you follow, and status updates.
To write a status update, type in the Status Update box on your timeline, and click Save Update. Updates are short: 140 characters on most sites. Mention someone with @ and their profile name, such as @admin, and they're notified.
Your followers see your updates and new items in their timeline. You see theirs in yours.
Some things on the site send you a notification: a new follower, a mention, an approved item, and so on. Choose which ones on the Notifications tab of Account Settings:
Which events are listed depends on the site's features.
Share Who You Follow, on the Account tab of Account Settings, lets others see the profiles you follow. Uncheck it to keep that private.