|Do I need to be "good" at yoga?|
I'm not sure there's any such thing. You don't need any experience and you are welcome no matter what physical limitations you may have.
Why should I learn the theory of yoga?
If you don't to, then you shouldn't! However, many people have expressed an interest in learning the philosophy, background and logic behind yoga, in particular as so many apparently conflicting views are held, that this area has become increasingly popular. More information can be found in my book, The Incomplete Guide To Yoga (available via Amazon).
Is yoga a spiritual practice?
The answer to that question could fill several books. The short answer for current purposes is that you can use sessions for whatever you want to use them for.
Do you know what you're doing? Why should I listen to you?
You probably shouldn't. But maybe working through sessions together will help you listen to yourself, which would be much more useful.
I have a normal amount of training and experience as a yoga and meditation teacher, with a probably above-normal amount of theoretical and philosophical training.
Do I need any "stuff"?
Not unless you want "stuff". The basic requirements are a space big enough to fit your body into, and clothing that enables you to sit down comfortably. Which really only rules out doing sessions from inside a wardrobe or wearing a diving bell.
What type of yoga is it? Do you follow any particular school? What are the types of yoga anyway?
There isn't enough space here to explain all the different types of yoga and what they're like. (If you want to know, book a Skype session to find out!) The short answer here is that it very much depends on what you want to use the sessions for. All physical yoga is adapted to the individual, so there is no risk of a practice that is not suited to you, physically or energetically. Unless there is a specific reason for practising in a given style (for example, you wish to have additional coaching in the style of group classes you attend), physical yoga practice is based on the YogaMonks method. Meditation practice draws on a variety of techniques, but is largely in the Zen Buddhist tradition.