Most people I know have a select number of apps that they use constantly while their Mac is open and on. Mail, Calendar and Messages are among the list of those that seem to always be open for me. As it is, most people spend the first minute or so after logging in to their computer manually opening each of the apps that they use all day long.
Wouldn't it be nice if there was an easy way to have the Mac itself open these apps for you? It is, after all, a computer, and computers are great at routine, repeatable tasks. That's what makes them so useful. It just so happens that there is a way to automate this ritual... and it's super-simple (as you would expect)!
First, we need to open our Users and Groups settings by going to System Preferences > Users & Groups.
From here, we will click on the "Login Items" tab at the top. The resulting list will show us what items are already designated to open when we log in to the computer. To remove an item, simply highlight it and click the [-] button at the bottom. This does not delete the app itself, but removes it from this list (meaning that it won't automatically open the next time you log in to your computer).
To add a new application to this list, click the [+] button, and select the app from your list of installed apps. If you want the app to open in hidden mode (meaning that the app is running, but minimized to the Dock), select the "Hide" box to the left of the item.
Once you are done building your list, it may look something like this: