How do I stop my Portable Computer from deleting the next letter when I start to type something?

It sounds like your keyboard might be in "overwrite" mode instead of "insert" mode. You can usually toggle between these modes by pressing the "Insert" key on your keyboard. If your keyboard doesn't have an "Insert" key, try looking for a function key combination, such as "Fn" + "Insert." If the issue persists, check your keyboard settings in the control panel or settings app.