The best place to start is with the next bugs you encounter. Making tests for those bugs will stop them from coming back in the future.
Then, any new feature you program should have its tests too. If you have a very strict budget, test the regular case and an edge case. You'll at least know the feature works.
After this, of you have time and money to do so, you can write tests for the older code that hasn't encountered bugs yet.
That's the way I brought unit/integration tests at my job, even if we didn't have budgets specifically for tests. This has led to fewer dumb or recurring bugs on our most recent sites.
Good luck!
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The best place to start is with the next bugs you encounter. Making tests for those bugs will stop them from coming back in the future.
Then, any new feature you program should have its tests too. If you have a very strict budget, test the regular case and an edge case. You'll at least know the feature works.
After this, of you have time and money to do so, you can write tests for the older code that hasn't encountered bugs yet.
That's the way I brought unit/integration tests at my job, even if we didn't have budgets specifically for tests. This has led to fewer dumb or recurring bugs on our most recent sites.
Good luck!