I need to change the salesforce credentials from myself to a different user. Are there any considerations I need to take before doing so? Do I just ask the new user to go and enter his credentials?
You could run into permissions issues if the new user has a different level than the current user. If you have any SFDC workflows set up specific to your user acting as the sync user, you'd also need to re-examine those.
However, what you should really have is a sync user used ONLY by Marketo. Not a human, not any other integrations, just Marketo. Having multiple users/systems use the same user makes it hard to control granular permissions, track API usage, and audit changes.
Retrieving data ...