Point my domain at my host

My site is <stack> on <host>. My domain is registered with <registrar>.

Tell me exactly what to set up:
- Which DNS records to add, where, and what goes in each field.
  Does the name field want "@", "www", or the full domain here?
- Whether to point the domain at the host's nameservers or add records
  at the registrar, and which is simpler for me.
- Making both example.com and www.example.com work, with one redirecting
  to the other.
- A command I can run to see what the world sees, without waiting.

If email is involved, tell me which records I must not break.