There are a lot of words that we will formally introduce this year in kindergarten. Many of them are decodable (meaning they can sound them out) but others are irregular and we call those heart words.
Making a word wall at home on the back of a closet door or magnets on your fridge, etc is recommended. Doing scavenger hunts, playing games like "Go fish", singing songs and writing the words in a sand tray or shaving cream etc are all great things to help them learn these words.
September
a
the
am
at
October
November
December
January
February
March
April
May
Some great sight word apps are: Kids Learn Sight Words (1-25) by Teacher Created Materials, Sight Words Ninja and Sight Words by Alligator Apps.