How to:set a public account

Hi,
I want to create a user for public that he can only use little functions.
below are my require for "him" to not having:
1.He can not modifies his account.
2.He can only see the "share-file" on the headers

In one word,the public user can only download files and scan the files on the site.

If anyone would like to help me.please tell me the place where the files to modify locate.And more likely