Start with a more rigid nib. It’s more likely that you write through your paper than destroy a Nikko G for example.
If you apply to much pressure, your nib will spring: you’ll notice that when it’s loaded with ink, but the ink won’t get onto paper or when the tines aren’t aligned in a way anymore so that ink can flow, it’s gone forever.
Calligraphy usually looks already pretty elegant without really fat downstrokes.
With a medium amount of pressure you can’t go wrong with most nibs. If you’re inexperienced, it’s better to use more rigid nibs like the Nikko G oder the Zebra G, or the Brause Steno.
If you apply too much pressure, sometimes the ink comes out all at once in a blob, or sometimes it won’t flow at all and you get the railroading effect. Also, your hand may tire sooner.