How long are you in France for? It's a bit of a nightmare swimming through the whole process of being self employed, especially if your French language skills aren't the best, as the documents and information you need are only available in French (they definitely aren't available in Welsh or Swahili or Tyke or anything, like in the UK). There's a wealth of information online though, and especially in areas with a lot of anglophonic people, you might find someone willing to help with the brunt of the official side of things for a small fee.
I just about stumbled through it myself with limited French language skills, but I wish I'd invested a little in someone willing to help, as it would have made the whole process easier. A heads up: tick the wrong box on the wrong form, or ignore one specific form you get sent out of a huge pile of them that you CAN safely ignore (because they are just scam artists trying to suck money from you), and you might wind up with no health insurance, which may or may not turn out to be a big deal later on down the road.
Once you are in the system though it's fairly simple, you can just log in to your account on the website and declare your earnings, and it'll calculate your contributions and tax there and then.
Most potential employers these days will not even look at you if you aren't auto ent, there's very little work to be done in the black unless you know the right people. A lot of the time employers prefer that you are self employed instead of on contract, because the contributions paid by employers are absolutely huge here.
Have a google for "auto entrepeneur advice English" and trawl through what you can find, it's pretty slow going but the more knowledge you can soak up the better.
EDIT
Oh, and just to add, you don't need a visa right now, and you haven't since we joined the EU. With Brexit looming however, it's going to get a whole lot more difficult very soon, and no-one has any idea what's going to be necessary in the very near future. So, you know, thanks to everyone who voted for that. Big help.