No one ever asks for a "certificate" ... at least in the US. In fact, if you go in with a certificate in hand, you could even be laughed at ... it's a sign of a rank amatuer who took some funky class. It really does not impress anyone in the biz to be honest. Sometimes a class can give you an understanding of what you will need to do ... but that's about it.
What they want to see is a dynamite protfolio, a strong resume and great referrences ... as well as an aesthetic that matches what they are looking for.
If you are just starting, and wanting to assist ... you must find a way to impress upon the stylist that you are willing to work hard physically. They won't really care if you have a fashion background or not. An assistant is there to do the grunt work, not any actually styling.
It's actually just hard labor ... carrying and lifting things and walking long distances with them and taking care of all the garments... pressing and steaming them. And you must be punctual, relieable and must learn to keep your mouth shut on shoots. You will probably be running errands a lot, so a reliable car is necessary.
What you will get in return is being able to watch the process unfold and maybe, with time the stylist might teach you a few things. And if he/she trusts you ... by not networking with his/her contacts they may eventually throw you a bone.
This is usually freelance work ... so you could actually assist more than one stylist at a time ... should you get lucky enough to impress a few of them. You will be called when needed ... with probably just a day or two notice and then the hours will be long ... up to 16 hours in a day at times.
Getting into styling is done by building a portfolio, working free to build your portfolio and hopefully working as an assistant and paying your dues. And by networking like crazy. People will hire you for gigs if they know you (or you have come recommended), like you and like your portfolio ... plain and simple.
If you read the previous pages (all of them), you'll see that I and others have pretty much outilined how we and others got into the biz. If you want to be a stylist, you must start to do your homework and reading all you can about it here is where you can start. If someone doesn't have the patience to read all of this information, then they don't have the attention to detail, and the ability to work the long, hard hours that it takes.