MWO is currently in open Beta and is "free to play" - although the only options you get are Trial Mechs if you spend $0.
First step is to go to the MWO Site, create an account and download the game.
Once you login you will have four Trial Mechs to choose from. These are not always the same, they have rotated a few times in just the short time I have been playing, but each class is represented: Light, Medium, Heavy and Assault.
I would stay away from the Assault, and maybe even the Heavies at first. There is a lot to take in when you first start. The Lights and Mediums have fewer weapons but are more nimble on the battlefield. There isn't a simulator to try to get a feel for the controls - you have to learn as you go under live fire. It is a painful, humiliating experience ...
However, there is a video tutorial that everyone should watch before they hop in that is very helpful.
A few words of caution about this game:
- Expect to get HOSED.
- You can shoot your teammates. This is a weapons free environment - you can accidentally blast your teammates, so watch that field of fire. Targeting is a lot of hand-eye, except for the weapons that will lock and follow.
- COVER actually works. Hiding behind something is a good way to avoid fire.
That said, it isn't just a free for all. There is definitely a rewarding level of detail in how you can manage your Mechs, a central team play component, and very legitimate tactical aspects.
About the team play aspect in particular - if you go off on your own you will get killed. You must work with your team to survive. I like this about the game.
I also like the tactical aspect of it, which I will get into more later.