Q. What condition should it be in?
A. Your phone needs to be in decent working condition, have no more than mild cosmetic damage, its original battery and be able to be switched on.
If it's only slightly under the weather, check how much you'd get for a fully working model. Often all it takes is replacing the battery, so it may be worth doing that yourself to get the decent sales value.
As a rule, you don't usually need to provide the charger but most will recycle it properly for you (though you could keep it as a spare or flog it on eBay).
Got a damaged phone? Most providers will consider non-working handsets, offering a reduced price for these (expect to get around 10-50% less) or at the very least will re-cycle 'em for you.
Typical damage which might mean you'll get less whack include badly damaged casing, a locked pin or failure to power up. Water damaged and broken phones with unresponsive or cracked screens will probably get zero cash. Make sure you click the 'damaged' box when searching for the best quote.
This can usually be found by removing the battery and will be printed on the back of the handset.