chmod [who=permission] [path]

Eg:

who can be u[for user], g[for group], or o[for others]

permission can be r, w, x or a combination like rw, rwx

path is the path to the file

Shortcuts:

Add write permission for other:

chmod o+w path

Copy permissions:

If you want to give the other, the same permission that user has:

chmod o=u path

Note that chmod by numbers is binary:

User Group Others
rwx rwx rwx

Eg:

u g o
rwx rw- r–
111 110 100

Convert the binary to decimal. The following links explain how:
http://www.wikihow.com/Convert-from-Decimal-to-Binary
http://www.wikihow.com/Convert-from-Binary-to-Decimal