The first thing you should always do to any highly compressed footage before you start processing it is to transcode the footage to a lossless or nearly lossless format. Moving up to 10 bit color will also help. Adobe Media encoder will go a good job at this. For DSLR footage there's Magic Bullet Grinder. I'm on a Mac mostly so I'm usually going to Pro Rez.For heavy effects work I also transcode P2 and XDCam footage.
This will give you a better chance at having things like Roto Brush work well.
If your camera has HDMI output and it will actually deliver a true HDMI full frame output you can capture directly to production formats using an external recorder. They start at about $350 and go up from there.
Thank you for your prompt answer!
I got M2t files already on my computer with Windows 7 and CS5.5. So, can you be explicit on the type of file I need to convert my M2t to?
It depends on which codecs you have installed on your computer. You can also download Black Magic's 10 bit codec for free. Please take a look at the rendering and output section of the help files. You'll learn more about the whole world of rendering and output.