Heathrow to Piccadilly - on to Kings Cross Station

Thank you!!! I will keep proper names -- I do not want to be confused.

What maps are you trying to read?

The tube map is very straight forward. Use the tfl.gov.uk website to help you plan routes to the
“ sites.”

What are you doing with your luggage?

Great city. Enjoy.

There's no left luggage at Kings Cross. There is one at the St Pancras station next door but it's quite expensive.

omtara,
"I am still figuring out how to read the maps." - "The" tube map is here: http://content.tfl.gov.uk/standard-tube-map.pdf
As you can see, the Piccadilly Line (dark blue) runs from Heathrow (square E1, bottom left), via lots of stations including Piccadilly Circus (square D5), via King's Cross (C5) and onward to the northern suburbs.
This map is the most important one you need to navigate around London, even more so than one that shows streets. Most sights, hotels etc. list their nearest tube stations.

2nd most important map, this one that shows bus routes for main tourist destinations in central London: http://content.tfl.gov.uk/bus-route-maps/key-bus-routes-in-central-london.pdf

And you have already done your research about tickets for public transport in London?: Oyster Cards or "Contactless". If not please come back and ask.

Wow! Thank you everyone. Very helpful. No left luggage at Kings Cross? is there a cheaper option somewhere near? Thanks again.

I know I sound like a stuck record about this but do not shorten place
names in the UK.

Don't do it to names in other countries either! I once nearly sent a person on another forum almost 500 km in the wrong direction because of a misunderstanding due to a shortened name.

Allow plenty of time to leave luggage at St. Pancras left luggage. The line took 20 minutes at least when I was there on a Sunday around 11 am last September. Picking it back up went faster.

Thanks again everyone! I really appreciate your time and help.