The basic strumming pattern is to keep your strumming hand in constant motion up and down in eighth notes, but only strum the strings on certain beats.
Count every measure (or bar) in eighth notes: 1-and-2-and-3-and-4-and. Strum down on the downbeats (the counted numbers), strum up on the upbeats (the 'ands' between the downbeats).
Now strum on:
1 strum down hit strings - 'and' strum up avoid strings
2 strum down hit strings - 'and' strum up hit strings
3 strum down avoid strings - 'and' strum hit strings
4 strum down hit strings - 'and' strum hit strings
This results in the basic strumming pattern in which you strike the strings on: 1, 2-and, and-4-and.
Caren covers it in this tutorial, see lesson 3 in particular.
https://www.guitartricks.com/tutorial/1408
Lisa also covers it in this tutorial, see lesson 6 in particular.
https://www.guitartricks.com/tutorial/1914
You might want to watch all the way through those tutorials depending on your skill level.
Hope that helps!