It can be so confusing to know what to order for a set of french doors. And it's true, it can be a little tricky if you aren't familiar with all the different functions and such. French doors on the inside of your house will have a door that's fixed and the other operable.
The fixed door will need to have a latch, called a flush bolt or a more decorative one called a surface bolt (these bolts come in all kinds of shapes and sizes and finishes to match your existing door hardware), that you can unlock that will be located at the top and/or bottom of the door to keep the door shut in place. And when you unlock it, it will allow you to open the door. Depending on what you want the french doors to look like, you can choose to order a dummy function door knob/lever (these are available with any style of door knob or lever handle you decide on) so it matches the operable door. Some people decide it's not necessary to order a dummy door knob, as it doesn't function and is only mounted to the surface of the door, whereas others choose to order the dummy function either because they believe it looks nicer or sometimes people like it because then you do have a handle to pull it open when the flush bolt is unlocked. A good thing to note when ordering the dummy function is that some brands only ship one singe dummy door knob/lever, meaning you will only receive one for one side of the door. Look for double dummy, which will come with two for each side of the door. If you are looking for what kind of hardware to order for double closet doors, read this post.
For the operable door, it's most likely bored with a hole to fit the door knob or lever. Lots and lots of doors already come this way with standard door prep. For an interior french door you will want to order a passage or privacy function door knob or lever. A passage is a door knob that functions but has no type of lock at all, whereas the privacy will have a privacy push/turn button to lock the door knob in place for privacy. You will find privacy functions on a bathroom or bedroom door. It just depends on what type of room these french doors lead to and your preference.
If you are looking to find what kind of door hardware to order for exterior french doors check out an earlier post all about that here.