This is a very good first attempt. Interesting premise and some good descriptions.
Constructive Criticisms:
Hate to be the grammar nazi (since I'm not innocent when it comes to proper grammar), but you have commas where there
shouldn't be any, and you have no commas where there
should be one. Your punctuation is off and, at times, it fucks with the flow and pacing of the story. If you clean it up, this story and future one's will flow much more naturally.
Secondly, while the characters of Topher and Nikki start off as interesting and relatable, they soon sort of devolve into flat archetypes. Their dialogue becomes somewhat...stilted? Unbelievable? Definitely not natural. Try and consider how someone might react in the situation that you described. Nikki just watched Topher's arm self-heal in a fraction of the time normally needed, Topher is watching Nikki slowly expand in every direction; both are experiencing very, very unusual alterations, and while it's okay for them to be excited about them, a little more trepidation and confusion might make their dialogue more believable.
Overall, this is a decent start, and don't let my CC dissuade you from continuing to write; you definitely should. I believe you're just a diamond in the rough, so-to-speak. With practice, you'll definitely produce better stories and exert a stronger command over the mechanics of writing.
And as with anyone who produces content, thanks for sharing it with the community; very much appreciated!