There a few different options for validation. You can validate in your microflow using exclusive splits and the validation feedback activity, you can setup validation rules on your entity, or you can mark a field as required on a form.
Here are some helpful links
I would also check out the learning modules.
Hope this helps!
Edit: I also want to add that you can also use a nanoflow with exclusive splits and the validation feedback activity. I would be cautious if you go this route because their could be some performance hits if you are retrieving data.