Search the help for "Passing Hidden Inputs in Donation Form Fields" for a comprehensive list.

A few specific options:

To display only a specific donation level:
&DONATION_LEVEL_ID_SELECTED=xxxx

- where "xxxx" is the dl_id (donation level identification number)

To display only the User Entered level and have it autofilled with an amount:
&DONATION_LEVEL_ID_SELECTED=xxxx&PREFILL_AMOUNT=yyy

Where "xxxx" is the dl_id and "yyy" is the amount you want prefilled. No decimals are required if the amount is a whole dollar. The dollar sign should not be included.

To chose which page in the process flow is linked to:
&donation=root
&donation=form1

Where "root" goes to whatever the default is for the donation form and "form1" is the first page of the donation form (skipping the splash page).