Starting out in Manual modes

Hi,

After buying my first DSLR and getting a feel for the basic controls I decided to try and teach myself manual mode.  After much reading i decided the way forward was to put the camera in fully manual mode, choose my ISO first, then my aperture and then use the exposure meter to get a good corresponding shutter speed.  If my exposure was too dark or light i would then move the ISO or shutter speed up or down a stop to compensate for this.

I wasn't having lots of success so i arranged a 1-2-1 with a photographer who told me to scrap what i was doing and to do it this way - Put the camera in Aperture mode and see what recommended shutter speed the camera gives.  Then adjust the ISO until the shutter speed is in line with the longest focal length of my lens so for example if i was using a 24-105mm lens i would always try and get the shutter speed to around 1/125 (as there is no 1/105) or If it was a 70-200mm lens i should always try and keep the shutter speed to 1/200.

I know that the above would not apply if i am trying to capture movement but as a beginner is this the correct way to go about it?  Am i better staying away from the 'M' mode as he suggests?

Finally he told me to start shooting in RAW and i was quite amazed at the differences that can be made to a RAW image that are not possible on a JPEG image however if i dont tend to do much processing is there much of a need to shoot in RAW?  (He told me it's a good habit to have for the future)

Thanks for any help