Begin on your SUP on your knees hip distance apart, holding your paddle in front of you across your board so as you get up it is easy to then put into the water. (If you have someone with you you can get them to hold the board from the tail end to keep it steady for you when you get to your feet, then they let you go as soon as you're up and paddling.)
Then looking forward slowly come up to your feet either straight up or into a squat first depending how flexible you are...Keeping your feet hip distance apart you can bend your knees this lowers your center of gravity...as soon as you are UP put your paddle in the water and start to paddle this gives you like a 3rd leg which helps with your balance plus the momentum will also balance you out.
Begin on a FLAT lake, ocean, inlet or river with NO WIND to begin...This way you'll get your balance sorted, get a paddle stroke happening and can get in some turns and have FUN and not fall off so much...I get everyone to do the Hula on their boards and to Laugh as this relaxes you....Remember when you do fall don't try to stop yourself by running either forwards or backwards as you may hurt yourself, just fall AWAY from from your board.
THE BIG THING is to LOOK UP to where you are going as Your Attention goes where Your Intention is AND then Laugh out LOUD because Your are SUPin'!
Board size, length and width are all going to play a part as well...1st timers definitely go bigger and wider this way you won't be falling off, getting tired and unhappy :)
Remember Be Safe out there in the water Always paddle with a friend and leash up. A lesson is always a good way to start any new Adventure
EnJoy and Aloha