For English, saying I am getting ready would mean it's something that you're currently doing right now. I get ready would imply that it's something you do repeatedly, for example you could say "I get ready every day before I leave the house". You wouldn't really say I get ready by itself as it sounds a little strange. There isn't really a direct parallel in Romance languages that I'm aware of, but it's sort of like a present-tense version of the imperfect tense. I am getting ready is probably me estoy preperando in Spanish, the only translation I can find for I get ready is me preparo.

"I get ready" is not a correct way to say it. You could say "I am getting ready" which means you are talking in the present tense. For past tense you could say "I already got ready". For future tense you could say "I am about to get ready" or "I am getting ready soon" I hope this helps.