how do i put a "lock" on a folder/file?

[sincerelyydavid]

how do i put a "lock" on a folder/file?

[asmoore82]

Right-Click => "Properties" and the "Permissions" Tab.

From there, you can set files to "Read Only" or directories to "Access Files[Only]"

[falconindy]

A stronger lock would be to set the immutable attribute (as you can still delete read-only if you own it):

chattr +i <target>

Unset it with -i.