First off all, congrats on the attention and the followers! A sudden influx of popularity can be a little overwhelming at first for sure.
If you want to quickly and easily sell prints of your fish, you can do it through websites like Society6. You just need to make an account and upload a high-resolution image file, and they will take care of printing it and mailing it to the customer for you. They can even print it on canvas, pillow cases, cell phone screens, whatever! They do take a cut of the profits, but especially if you don't have a quality printer and shipping supplies yet, it would probably shake out. If you get to the point where you're doing a LOT of prints, then eventually it will be more cost effective to get your own color laser printer (which is more expensive up front than an ink jet printer, but you save so much money in the long run by not having to buy overpriced ink) and a Stamps.com account for easy and affordable shipping.
If you do get to the point where you're shipping your own prints, always make sure you mail art in a sturdy cardboard mailer, sandwiched between layers of cardboard or chip board, so it won't get bent by the postal service. And no matter how many "do not bend" stickers you slap on a package, if it can be bent, it will get bent. And ship with tracking every time so shady customers can't claim they never received their order - USPS first class with tracking is under $4, and is worth it.
As for commissions, the most important thing is to price yourself fairly. Don't undersell yourself, it diminishes your own value and makes it harder for other artists to claim their own value if everyone else is undercutting each other. Charge AT LEAST minimum wage. If it takes you eight hours to do a picture, multiply that by at least your minimum wage.
When I'm doing a commission over $100, I require half the payment up front before I even start a sketch. Then when the image is done, I'll send it to the client with a big obnoxious watermark on it. After they send me the other half of the payment, they get the full-sized unmarked image to do with as they please. I only do very minor revisions, and only if they ask nicely. If they forgot to include something important in their request, that's on them, not me.
If you have any hard boundaries about content you are not willing to draw, stand by that and don't compromise yourself if someone tries to push back on that. Your boundaries are worth more than their money.
Best of luck to you!