The common elements.
WHO
The parties, named properly.
Business names as registered, if registered.
WHAT
What is being provided, specifically.
Vague descriptions cause most disputes.
WHAT IS NOT INCLUDED
The exclusions. The most valuable section for preventing disputes.
WHEN
Timescales, and what happens if they are not met.
HOW MUCH
The price, what it includes, and when it is paid.
WHAT EACH PARTY MUST DO
Not only your obligations. Theirs too.
Frequently a project depends on the client providing something.
OWNERSHIP
Who owns what is produced.
CONFIDENTIALITY
What may not be disclosed.
HOW IT ENDS
Notice, and what happens on termination.
WHAT LAW APPLIES
Relevant if parties are in different places.
WHAT TO TAKE ADVICE ON
The specific wording, for anything substantial.