Particular resources and guides are gonna depend heavily on language/frameworks/etc.
Start with areas that are the most valuable to the business. Not only does this more easily show the (business) value of testing, but it helps keep you from introducing a very visible and very bad looking regression. Anything dealing with money (especially billing) is probably a good indicator. Be warned, here be dragons.
That being said, don't verify your testing setup with business critical tests. Verify your setup (to yourself and other devs) with the quickest thing you can accomplish. The business critical ones are your (team's) short term goal.
Pick out the X most important logic paths (e.g. customer checkout or some subsection of that). Work on regression testing that over the next few months.
Make sure your tests are both automated and actually get run. The latter is more important than the former.
Ideally, your tests will prevent a nasty bug from going in and will immediately prove the business value of your testing efforts. This will establish the trust you need to continue testing and refactoring.
Start with areas that are the most valuable to the business. Not only does this more easily show the (business) value of testing, but it helps keep you from introducing a very visible and very bad looking regression. Anything dealing with money (especially billing) is probably a good indicator. Be warned, here be dragons.
That being said, don't verify your testing setup with business critical tests. Verify your setup (to yourself and other devs) with the quickest thing you can accomplish. The business critical ones are your (team's) short term goal.
Pick out the X most important logic paths (e.g. customer checkout or some subsection of that). Work on regression testing that over the next few months.
Make sure your tests are both automated and actually get run. The latter is more important than the former.
Ideally, your tests will prevent a nasty bug from going in and will immediately prove the business value of your testing efforts. This will establish the trust you need to continue testing and refactoring.