I'm just sat here now before I have even bought my D-SLR yet and I was just wondering how I would know whether a lens would fit onto the front of my camera once I buy it. What I mean by fit is , will it screw on or will it be to big, I guess I mean the circumference of the lens rather than the depth ? . I'm racking my brains trying to work our the answer to my own question but don't have a clue. I'm such a newbie !
Charlotte, do you mean different lenses, or do you mean screw-on filters?
Lens
Each manufacturer usues their own mounting system, sometimes more than one (but not many do this) but sometimes there are subtypes.
If I take Canon as potentially the most confusing (hopefully someone will chip in later with Nikon and someone with Sony, Olympus, etc).
All Canon DSLRs use the EF mount. So any Canon EF lens will fit any Canon DSLR.
Most Canon DSLRs use the EF-S mount (a sub-type of the EF mount) and will take any Canon EF lens and any Canon EF-S lens. These are the most common bodies and include the 1000D, all the XXXD bodies (e.g. 500D), all EOS XXD bodies (e.g. 50D) and the 7D. If you stick to these bodies then any recent "Canon fit" lens will fit.
The only ones to watch for with Canon that won't fit are the old Canon FD lenses that crop up very cheaply on Ebay, etc. As long as you always look for EF or EF-S you'll be fine.
All the above only applies to Canon, this is the bias of my experience. I'm not familiar with Nikon etc which all have their own quirks (perhaps someone who knows such things can explain the Nikon focus designations for lenses as this is very relevant to entry-level models like the D3000 and D5000).
Filters
As you mention the diameter at the front.. could it be you're thinking about screw-on filters? the diameter of the filter thread is usually written on the front of the lens, or if not it's on the side of the lens. It's also written inside the lens cap. As long as you get a filter with the matching millimetre diameter it will screw on the front as the thread is standard for all manufacturers.
PS
+1 on ignoring the false promise of cookies from the Nikon camp. Yes, there are what appear to be jaffa cakes on the plate, but they are cheap Morrisons immitations and they've sucked out all the orange.. it's just a hollow chocolate shell of a promise..