Don't assume everything that looks different and seems strange is a Mint Error. Post your coins at the Error Coins forum of CoinTalk. First find out first if it is a true Mint Error or not... worry about 'what's is worth' last... most beginners have damage coins.
That nickel has had a rough life. Everything you see is called PMD or Post Mint Damage. The X on the dome was done by someone like you or me after it left the Mint. A spender for sure. Welcome to CT.