If it is valve related, it is most likely power amp tubes. Preamp tubes basically last forever...at least they last a very long time. People who replace them are looking for a tonal change primarily. Power amp tubes are the ones taking all the current.
To do this properly, you'll need a multi-meter so that you can get the bias right. You'll also need to know what the -mV bias settings are for your amp as recommended by Marshall. If yours is a 100watter, then I think it is -90mV per side, but please check that with Marshall....
On the back of the amp there will be a little white dial and a three prong connector. The dial is the bias adjust, the prongs are where you connect the multi-meter. The middle prong is neutral, the two outer ones are hots. Black lead goes to the middle, red lead goes to which ever side you are adjusting at the time. In order to adjust properly, the amp must be powered on and in "ready to play" mode...ie, not in standby, and you must have a cabinet connected to provide proper load.
If you don't understand any of this, you'd better take it in somewhere and have a tech do it. It's really not hard at all...but you need to understand what you are doing or you risk screwing it up and messing up the tone of your amp by running it cold, or shortening the life of your new (and not cheap) tubes but setting it too hot.