Puja is a means to think of God. It is a set of procedures one could use to worship ones favourite devata (ishta devata).
Who is your ishta devata ? Do you have a preferred image or picture of your favourite devata ? The puja procedures (Upachara) is what one might do for a visitor to our house. In this case, ones ishta devata is the visitor and ones mind or ones chosen physical image is the house and the doer of the puja is the host.
There are variations to these procedures. One should give preference to ones family tradition in adopting the procedures to do Puja. A form of puja is the Shodasha Upachara Puja.
What is Shodasha Upachara puja ?
(Shodasha = 16; Upachara = service) It is a 16 stage means to worship. The 16 services are: [*]
Inviting the devata (Aavaahanam) – I usually imagine the devata coming and entering the image (in front of me) at this time, Offering of seat (Aasanam), My ishta devata is Vishnu so I imagine seating him in a chair, Offering water to wash feet (Paadyam), Offering water to wash hands (Arghyam), Water for inner purification (Aacamanam), Bathing (Snaanam), I imagine pouring pots of water on Him.
This is my central upachara. I spend most time on this. Offer of clothes (Vaasam), Offer of sacred thread (Upaviitam), Offer of sandal paste (Gandham), Flower offering (Pushpam) also known as archana, this becomes the central upachara during special pujas on days like Makara Sankaranti, Rama Navami, etc. Incense offering (Dhuupam) Agarbatti / Saambraani offering, Show of lamp (Diipam), Food offering (Naivedyam).
Anything that one cooks & eats for the day, Saluting by falling flat on the ground (Namaskaaram), Going around clockwise (Pradakshinam), Saying Goodbye (Varjanam).
There are variations to the above sixteen.