Virmire is the third main planet mission.
You will start in your Mako and will go through a long range drive with many Geth throughout. If you aren't hard on experience you can drive past all of the enemies, except when you must stop at a couple outposts along the way to open the gate to proceed.
After this section you will come upon the Salarian Camp.
At this point you will decide whether you will kill Wrex or not (you will need Charm at Level 10 to "save" him), after this event, you will also decide who to take with you and who will go with Captain Kamehe (Ashley or Kaiden). There is also a merchant in a tent to the left of the Captain if you need some items.
The base if very straightforward consisting mostly of Geth, drones, and some Krogans. You will then come to a large open area with water where a nuke will be planted, continue forward and an event will happen
You will have a choice of going back to whoever you brought with you (Ashley or Kaiden) or saving whoever you didn't take with you from the AA Tower (Ashley or Kaiden) depending on which team member you go back to, the other will die regardless.
Shortly after this you will have a quick firefight with Saren, after you defeat him, the mission is complete.