At the most basic level you have only two options on the guitar. Strom up and strum down (that applies to both single strings [alternate picking] and group strings [strumming.]
If you are using a plectrum (flatpick) start by simply going up and down at different speeds and then add some personality to it by making soft and hard strums to generate accents and give texture to your song. Remember that pauses are as important as tones. Avoid sounding like a robot. Give some soul to it (that only means sound human and not mechanically boring.)
Lets say you are experimenting with 4 strums. Start with strums down only. It should go "1, 2, 3, 4, 1, 2, 3, 4, etc."
Now add some "strums up" in between numbers. Those will be your "ands". You should get something like "1 and 2 and 3 and 4 and 1 and 2 and 3 and 4 and..." numbers are "down strums" and "ands" are "up strums."
Practice this with single strings and groups of three or four strings as well.
Next step is doing as many variations as your imagination can handle, change not only the strumming but the accents (strumming strengt) on each pattern...
a) 3 down-1 up (repeat)
b) 2 down-1 up-2 down-1 up (repeat)
c) 3 up 3 down (repeat)
c) 2 down 2 up (repeat)
and then try to combine, for example:
d) 1-2-3 • 1-2-3*• 1-2 • 1-2-3 • 1-2-3 • 1-2
...the possibilities are endless.
Practice first with the strings muted. Don't worry about the cords for now, those will improve naturally once your rhythm hand is up to speed and works almost by itself without supervision, just concentrate on the rhythm hand until you are comfortable with your own favorite patterns.
Also jam against some records, they are more fun than a metronome for practice. I bet you can replicate those rhythms very easily once your have your own personal strumming down. Good luck.