New tubes in a Port City Pearl?

[rshull07]

Ive had my pearl for over a year now and I believe its time to retube it. Theres a tube rattling like crazy, and Im getting a really high pitched faint whistle through the amp when Im not playing. As well as the amp is just sounding a little flabby. Ive never put new tubes in an amp before, Ive been told its an easy DIY job but Im not too sure. Any pointers? And anyone know where I can get a full set of tubes.

Thanks