Gina,
There are a few options depending on how you want the system to work. See a list of related articles here:
http://www.marketingrockstarguides.com/marketo-form-instructions-164/
Prepopulation happens by default on all Marketo forms.
To hide fields that are filled in already, you need to enable Progressive Form on the forms you create. These will all work using iFrames.
There are more complicated ways to embed Marketo forms into your main website using the API, jquery, or double post method. Those will require programming expertise.