Hi JillB. Welcome to the forum!!
Guppies drop their fry anywhere from 21 - 30 days with the average being 28. Can you check your receipt or account to see what day you purchased her and count it out to today. Are you close to those dates yet?
What size is the tank and do you have any live plants in the tank? Even if you don't a large leafy plastic plant can be floated on the surface to give shelter to the fry when born. They will hang out at the surface for the first day at least and that is why they can get easily eaten.
The other option is a birthing or nursery box, they come in netting and hard plastic. There are advantages and disadvantages to both. The trick is knowing what day or evening to put Momma in it. Again, count your days to see where she might be at.
If you get the box but don't get the opportunity to put Momma in it, try to check the tank each morning without putting on the light, use a small flashlight as turning on the light and peering in the top attracts the other fish and they will notice the babies. Drop in some sinking food if you have it and while the other fish are "busy" you can net the babies if they are loose in the big tank and move them into another tank or the nursery box.
Don't hesitate to ask any questions you may have. Hopefully I haven't confused you.