Does activating the item require the user to do something special?
Yes, for items you need to activate the effects of
You can't activate its effects unless you know how to use the item.
To learn how to use it you need to do one of the following:
- Spend a short rest focusing on the item. This takes 60 minutes (unless resting rules have been changed). (SRD v5.1, page 206; DM's Basic Rules v0.5, page 58)
- Cast identify on the item. This takes 1 minute. (Basic Rules v0.3, page 94)
- Get instruction from someone who already knows how to use it. Depending on how many properties the item has and how complicated the procedure to activate them, this might take a few seconds or many minutes. Depending on the activation method, observation of someone using the item might be sufficient. (Not mentioned in the rulebooks, but seems obvious to me)
E.g. Most wands, ring of evasion, bag of tricks, stone of controlling earth elementals.
No, for items that passively grant benefits
You gain the effects from simply using the item as normal (depending on attunement, see below).
E.g. Weapons, shields, armor, cloak of elvenkind, gauntlets of ogre power, ring of free action.
For items with both passive and active effects
If an item has passive abilities and has abilities that require actions then you only gain the passive ones (depending on attunement, see below).
E.g. If you are using a staff of striking without identifying it, then you will have the +3 attack and damage bonuses, but you won't be able to expend charges.
Using a non-attuned item
For an item that requires attunement, you only gain the non-magical benefits while not attuned. You can use a magical sword, but it will not give any attack or damage bonus, and will not count as magical for the purposes of bypassing resistance. You can use a magical shield, but it will only give +2 AC. You can wear a robe of stars but you won't get a saving throw bonus or be able to cast magic missile. (SRD v5.1, page 206)
Using an attuned item without identifying it
This is not explicitly mentioned in the books. The SRD says that the short rest used for attunement cannot be the same rest that identified the item, but says nothing about the order of the two rests.
I think both "you can't attune to an unidentified item" and "you can attune to an unidentified item, but you will only gain the passive abilities" are fair.