You may need to contact a vendor, creator, owner, or pretty much anyone for a variety of reasons.
If they are within visual range, you can right click on them and choose [Send IM…]. Type your private message to them.
If they are not in visual range but you have a name, click on [Search] at the bottom of your screen, enter their name in the [Find:] field of the [All] or [People] tab, and click [Search] (or press <Enter>).
On the [2nd Life] tab of their profile, click [Instant Message…]. HOWEVER, please read their profile first. IMs cap after only a few in SL which means your message may not reach them if they are not online. Vendors and creators and popular people generally prefer you to send a Notecard (see below).
You can also bring up the profile of someone by right clicking an item they own or have created and then clicking [Edit…]. Then click on [Profile…] next to Creator or Owner on the [General] tab (you may have to click [More>>].
You can also send IMs to someone in SL from the Internet (if you are not in world). Please see zonkster.com.
You may need to send someone an item (object, notecard etc.). Open your inventory so that you can see the item.
If they are within visual range, you can drag the item from your inventory onto the person.
If they are not in visual range bring up their profile (see above) and then drag the item onto their profile where it says [Drop inventory item here.].
Note the permissions of the item. Right click the item and choose [Properties]. The permissions are indicated under [You can:] and the check boxes checked. If [Resell/Give away] is unchecked, you cannot transfer the item to another. If [Copy] is unchecked, you will be giving away your only copy.