1.I worried if she can easily enter the country (Canada) without any visa? (Generally US citizens do not require a visa)
2. what are the forms and process for application for sponsorship would be for her?
3. will she be required to go through med examination? and can I take that exam here in my home country to have all the papers ready while sending the application?
1. Yes, she can enter but her status will be visitor so you must sponsor her for PR to allow her to continue to stay in the future. When you enter Canada, ask for a passport stamp or a visitor record for her because of your intent to sponsor her. A stamp will allow you to prove when she entered Canada in order to be able to extend her visitor status if the sponsorship takes more than 6 months. A visitor record can be issued to give her more time, up to a year.
2. The forms can be found here:
http://www.cic.gc.ca/english/information/applications/fc.asp Because she is a US citizen, you can apply through Buffalo which is relatively fast.
3. There will be a medical examination but you can have it done anywhere you want, in your home country or in Canada. You can not send the application until you are in Canada because a PR must be in Canada to sponsor.