I agree with all of the above:
- Do some tutorials
- Attend the user groups...
Then dive straight in! Organise some test campaigns to built up confidence and keep the community pages open. There are so many people who are happy to share their knowledge and experience and if you don't find an answer to your question straight away, you can start your own discussion, be as detailed as you can and get all the answers you need from experts, consultants, Marketo staff and regular users who have had similar experiences.