meter vs poner
Poner places something on a surface. Meter puts something inside. 'Put' needs two verbs in Spanish.
meter
To put inside — into a container, a space, a situation.
Mete la ropa en la maleta.
Put the clothes in the suitcase.
Into a container. The preposition 'en' marks the place.
Se metió en la cama.
He got into bed.
Meterse = to get into. Also 'meterse en problemas'.
¡No te metas en mis asuntos!
Don't get involved in my business!
Meterse en = to interfere. Also 'meter la pata' = to put your foot in it.
poner
To put on a surface, to place or position something.
Pon el libro sobre la mesa.
Put the book on the table.
On a surface. Preposition 'sobre' or 'en'.
Ponlo ahí.
Put it there.
General placing. The position is open.
¿Dónde has puesto las gafas?
Where have you put your glasses?
Irregular past participle: puesto.
Tricky bit
Both can translate 'put', so check the destination. Is it inside something? Use meter. On top of something? Poner. 'Pon la llave en la cerradura' is fine, but 'mete la llave' stresses the insertion.