It is possible to upload several kinds of file to Second Life, to create your own items. To upload files, go to the top menu, Build → Upload, and select the type. A file selector window will open; use it to navigate your file system to locate the file(s) to upload 1).
The following upload file types are available:
There are two more items in this menu:
Note that in most cases, you will be charged 10L per file uploaded, by SL. This is debited to your SL account. Therefore you must have at least that much on your account to be able to upload.