Is there any way of converting a illustrator file with text and logos to one only. As I'm working for a client that only wants to print with one-color?
With Illustrator CS4 or greater it's pretty easy.
Select all the artwork.
Choose Edit > Edit Colors > Recolor Artwork
If the artwork contains black which you want to alter as well..... Click the little pref icon next to the "Preset" drop down menu. This will bring up the Color Reduction Options dialog.
Then Proceed to #3 below.
If the artwork does NOT contain any black, simply choose 1 under the color drop down in the middle of the dialog window. Then proceed to #3 below.
Then double-click the little box under where it reads "New"
The Color picker will pop up... choose the color you want (you can use the basic color dialog or you can pick a swatch if you have a Pantone Swatch you want to use. Add the Pantone Color to the swatches before you start all this though.) Then hit Okay.
Since you say "print"...
The general rule I've found among providers who print single color (single plate, single silk screen, whatever) is that they prefer the artwork itself in black with a specification as to the desired color (usually a Pantone number).
There may be exceptions to this, but I've not run across any. You or your client should ask the print provider for guidance on how they prefer the work set up.
You can very quickly convert to "black plate only" in Acrobat, of course, but Scott's approach will work nicely inside AI.
This is supplementary info, but there's a bit you have to know about printing before you convert the artwork.
It sounds like you want your printers print a one colour job, i.e. use a single, pre-mixed green "spot colour" ink, rather than printing green tones with the normal CMYK process (cyan, magenta, yellow, black).
You need to specify exactly what green you want the printers to use. The normal way of doing this in most of the world is to choose a colour from the Pantone colour matching system. Can the client tell you the Pantone reference they normally use for this green? If they have a style guide, it should be in there.
If you need to pick the colour, all Adobe software will allow you to choose Pantone colours from a palette, but given you are dealing with ink, the reliable way of doing this is taking a look at a printed Pantone sample book. These are expensive, so unless you need to do this a lot, you'll probably want to borrow one, take a look at one when you meet your print rep etc.
(Just checking, there wasn't already a "what is a Pantone colour" question. So I've used this as the basis of one: What is a spot/Pantone colour?).