I'm not sure how to know which space I need to set a document in Adobe Illustrator CS5 when I want to create a six-sided DIN A4 folder-flyer. Is there a template somewhere out there that I can use? Or how can I configure the document so that the flyer will be I want to create this flyer and let it get printed by a printing shop, and we've only got Illustrator in our company - and I didn't work with Illustrator too much before. I hope this Stackexchange site can help me even if I don't have much experience with CS5.
In AI CS5, you can select
File > New From Template and under the Japanese section, in the Marketing folder, you'll find that j-brochure1.ait is an A4 template for a trifold. Just delete the artwork and keep the fold marks. If you have a European version of AI, you may find that the regular Blank brochure template is A4. On my US version, it's US Letter size, which wouldn't be much help to you. The folds are not evenly spaced, so there's an art board for the "outside" and one for the "inside" of the piece.
There are many companies that offer printing services to "the trade" and consumers through their websites. It's become a very common business model for companies offering standard commercial jobs such as postcards, fliers, booklets, etc. Every one I have seen has templates on its website available for download, and the kind of trifold you describe is a very standard item so you should have no trouble finding numerous templates. The print shop you are planning to use may also have one, so it's worth asking them. (Rule: Always ask your printer!)
A jpeg can be dropped into Illustrator and the artwork built on it, or you may find a native Illustrator or EPS template.
I'm going to concentrate on the practicalities and gotchas of this piece, rather than the specifics of Illustrator.
You'd normally refer to printed items by their finished (i.e. folded) size. So this would be a six page (or tri-fold) 99mm x 210mm flyer.
In the UK at least, this size is often referred to as "DL", although this is actually the size of the envelope that accomodates it: 210mm x 220mm.
There are several ways of folding this up. Try it and you'll find there are two obvious ones. These differ in page order and affect the way you lay out the artwork. I'd illustrate, but don't have drawing tools to hand.
Roll fold/gatefold is probably most common: with the flyer on a table in front of you, you'd open up the cover to the left, and then an inside cover to the right.
You could design this as six individual panels, or a three page spread (the inside) plus three single pages (front, back and inside covers). There's a "spine" at the left which makes the folded form easier to handle.
Z fold: stood up and viewed from the top, the sheet is folded in the form of a "Z".
You could design this as six individual panels or two 2-page spreads and two single pages (front and back covers). But there's no "spine" at the left, and so it's a bit more more difficult to pick up and handle. This layout might be useful when for example you need to show a pair of large diagrams - e.g maps for the two floors of a museum.
You should definitely make up a couple of draft examples and think through what you'll put on each panel befire you proceed.