You are doing the right thing by getting in to see your doctor
I doubt that you have torn your ACL. Although this is technically possible with walking and a fall, ACL injuries are usually very high impact injuries, such as those that are sustained during sports activities. If you have had a prior ACL damage, then this general rule may not apply however.
On the whole, it is much more likely that you have something simple like a knee strain or sprain. You also may have some meniscal strain or tear, which would be more likely than a ACL tear. When you go to see your doctor you should accurately describe how the injury
occurred, as well as point out any areas where you especially feel pain and what sorts of activities make the pain worse. Also describe to your doctor any sensation of looseness or 'give' in the knee. This information will make it easier to pinpoint the type of injury.
Your doctor will perform a full exam of the knee and will then make a diagnosis and recommend suitable treatment options.