My mg102fx has always been reliable since i got it just over a year ago... and today the sound coming from the amp fizzed and cracked, the volume changes from very quiet to the volume i have it set at. I hope its nothing too serious, and preferably something i can fix at home myself. It's my first high powered amp (i'm a 15 year-old) and it would be a major dissapointment if this marshall broke down on me considering it would take me a long time to afford another amp of this caliber.

Hi there
Firstly don't panic! It might be something quite simple - and it might not actually be the amp that is at fault.. Here are a few things you can try to start with to trace the problem by systematic elimination.

1. If you are using any effects pedals unplug them and plug your guitar straight into the amp - does that solve it? Turn off any built in effects on the amp and unplug the foot pedal.
2. Try a different guitar lead - leads are very prone to failure -especially at the solder joints in the jack plugs - but sometimes in the middle as well because they get pulled a lot in use. The tiny twisted copper wires in the core of the lead can become cracked, which will increase the electrical resistance (causing the volume to drop. As you move about the bits of broken wires may touch again (reducing resistance so the volume increases again).
3. Check the tightness of the screw threaded rings (bezels) that hold the jack sockets on to the body of your guitar and the front panel of your amp. These can work loose over time (especially on the guitar) and cause the jack plug to only make a partial contact inside the socket with the same results as no 2 above.
4. On both the amp and on the guitar quickly and vigorously rotate all the volume and tone control knobs backwards and forwards (up and down) fully several times - the copper contacts inside the knobs can become oxidised over time. Doing this will clean them a bit by means of the friction between the contacts. Do this with one knob then test to see if the problem has been resolved before moving on to the next knob - that way you will be able to identify which one is faulty (you may hear some crackling as you rotate it). In the same way flick your guitar pick up selector switch backwards and forwards several times. If you find this resolves the problem you might apply a VERY little squirt of switch cleaner ( a spray that you can buy from electrical component shops like Maplin) to give the knobs / switches a more permanent cleaning - but don't overdo it and allow a few hours for any surplus to evaporate before you use the amp again. HOWEVER - don't use switch cleaner if your amp is still in manufacturers warranty AND if rotating the knobs hasn't solved the problem.
5. If none of the above has solved the problem then the amp itself might be faulty. Don't attempt electrical repairs yourself. Apart from the obvious danger of messing with electricity you may actually do more harm than good and without proper test equipment it is extremely unlikely you will be able to trace a faulty component. If the amp is still in warranty take it back to the shop (or phone/email them if you bought it online). If it is out of warranty your local music shop may know of a good repairer in the neighbourhood who won't rip you off (they may even have their own 'techie' who can do it for you).