If you want to transition between various missions, the StarCraft 2 campaign guide will be your best friend. Switching back and forth between missions is often one of the ways gamers like to play. In the single player option you can do this simply by saving and loading and toggling back and forth at will. Battle.net will allow you to play and record your achievements and keep a record of your stats as well. You can play online or offline and get just as much out of both modes. The biggest difference between the two is that Battle.net saves your stats while offline play will not register.
The game has 29 different missions within the campaign. As you play you may need a StarCraft 2 campaign guide to show you exactly how you go about unlocking each mission, and of course will list the achievements. Each mission will fit nicely into one of seven story arcs. Each arc has its own order of missions to be completed, and you're not permitted to skip one and go onto the next. What you can do is play one arc for awhile and then go to a different arc and switch back and forth. The game is set up to have a few exceptions like the Mar Sara line. This you must complete in order to get access to other missions.
Once you have completed any given mission the information will be stored in the archives. You can find the archives on the Hyperion Bridge. If you missed any of the achievements you can return to the archives and replay a completed level. StarCraft 2 campaign guides will help show you how to find the archives, replay a mission, and jump from one mission to the next when permissible.
The Rebellion line hosts a secret mission. Once you complete the Media Blitz mission you will find a new mission called Piercing the Shroud. This is the secret mission. The very first time you play the Media Blitz mission make sure you destroy the science laboratory in order to open the secret mission. No one has been able to unlock the secret mission if they wait until a later time to destroy the science lab.
For the first mission you will find that it's really quite straightforward. You will be playing as Jim Raynor, the Rebel Commander. Alongside of you there will be 5 marines. The objective of this mission is to locate and systematically destroy the logistics headquarters in order to effectively free the people of Mar Sara. If you need help learning to play during this first mission you can ask for the pop up videos to display themselves. You will be able to see how right on the screen as you move through the mission. Just click on the pop ups and the video will load and play. You will receive instruction on maneuvers and the basic premise of attack within the videos. The videos can easily help accentuate a StarCraft 2 campaign guide right there on the screen as you're learning to play the game.