I think lie and lay are two of the most frustrating verbs in English.
lie (1) = not tell the truth (lie-lied-lied)
lie (2) = be in a horizontal position (lie-lay-lain)
lay (1) = place in a horizontal position (lay-laid-laid)
lay (2) = e.g. lay an egg (lay-laid-laid)
So in your example:
I often lie on the beach, (present)
I often lay my towel on the sand. (present)
Yesterday I lay on the beach. (simple past)
Yesterday I laid my towel on the sand. (simple past)
Have you ever lain on a beach? (present perfect)
Have you ever laid your towel on the sand? (present perfect)