If you're trading through a "good" broker, i.e. one whose online trading system has a modicum of smarts built-in, you will at least be told that the number of shares you want to sell exceeds your current holdings. And there are many more cross-checks that can be activated. If I were new to online trading, I'd ask my friendly advisor on setup to explain what safeguards their system has and please activate any that protect me from my own butterfingers or ignorance.
(And if they don't have any safeguards, ring somebody else that does.)