A good place to start is by documenting your current roles/responsibilities and any information relevant to each one. As an application developer that might not include too much specific information, but if there are any server instances that you manage you might want to consider listing the hostname, login details, configuration info, and similar information for each one.
Similarly, if you're the keeper of any important developer account passwords (or details for any testing accounts, etc.) you should ensure that they are properly documented somewhere and that the accounts are transitioned over to the company if necessary.
If your company has a password protected wiki that would be a good place for storing this information. Though check first in case they already have a policy about where this sort of information should be kept.
You may also want to discuss with your manager and see if they'd like you to set aside some time to help with interviewing and/or training your eventual replacement, or if there's any specific information they want to make sure you document.