You say "when I touch the mic with my hand"... are you trying to hold the mic? A condenser mic is not meant to be held, touched or handled when active. If its going to be moved around, it needs to be supported in a shock-resistant mount because its very sensitive to handling noise.
The Sterlin ST51 uses a "Cardioid Polar Pattern", which in pure laymen's terms, means it picks up sound very well from sources its pointed directly towards, and poorly from sources that are directly behind it. If you place the mic on a stand pointed at the ceiling, so that you can stand and sing into it, its going to pickup the amp fairly well to... you can try moving the mic as far away from the amp as possible, and tilting it away from the amp if possible.
Dynamic mics are far better for "rough & ready" general purpose use. Like the Shure SM58 for vocals, or the SM57 which is pretty much industry standard for miking guitar amplifiers. If you're using the mic for live vocals, then I'd suggest the SM58 over the Sterlin. Large condenser mics work well for recording acoustic audio sources in isolation... in other words, in situations where a single source (an acoustic guitar, an accordion, a choir) is the only sound in the room. Not so much for live PA work, if that's what you're trying for.
