What is a Matron Goddess or a Patron God?
A Matron Goddess is the Goddess that you are personally aligned with, devoted to, work with all the time, devoted your altar to, etc. She's your "go-to" Goddess that you feel a strong connection with. The same is true of the God, only substitute male pronouns lol. These deities are the ones that you have really formed a personal relationship and/or alliance with.
Do I HAVE to have one?
No! You do not HAVE to do anything or have anything in the Craft. If you haven't found a deity/deities that you feel a connection to, that's ok. It's perfectly fine to use Lord and Lady, God and Goddess, etc. during rituals and the like, and it's also perfectly fine to use different deities for every work you do. Just keep in mind that even if you don't use two deities from the same pantheon, they DO need to be complimentary. You don't want to mix aspects, even if you're comfortable mixing pantheons. You may never have a Matron or Patron deity and just work with all different deities throughout your journey in the Craft. But if you DO come across a deity that just really seems to call to you (like you dream about him or her, you keep coming across their name in reading, can't stop thinking about him or her, etc.), don't be afraid to work with that deity and see how things go. And of course, there ARE atheist witches out there as well.
How will I know if I've found the "right" one(s)?
You will know. I can't tell how to figure it out, but when it's right, you will know.
I think I found my M/Patron Deity, but I've heard (insert what you've heard here) about him/her.
I found out the hard way (haha through experiences) that when it comes to YOUR Matron or Patron Deity, FUCK what everyone else thinks and says! Sorry for the rude/blunt language, but it's true. My Matron is Hec(k)ate and I've heard all sorts of crap about her and my relationship with her. People were constantly telling me how you can't call on her unless you're in DEEP shit or you'll feel her wrath, and how she can't be a Matron deity because of that, etc. Bull shit! I've called on Hecate for many, many things and she has ALWAYS come through for me, and I've never experienced her wrath. Now, I haven't called on her for something like a broken nail, eesh but I work closely with her, my altar is dedicated to her, as is my BOS. I have found nothing but peace, understanding, love, comfort and knowledge (well, maybe some other things like those that I'm forgetting, also) from Hecate in the time that we've been together.
She is a Dark Goddess, and that scares a lot of people. I haven't had a single non-Dark and/or Crone Goddess call to me. It was Cerridwen before Hecate. Apparently I need to learn a lot lol, for knowledge is one of the greatest gifts of the Crone. If a Crone calls to you, don't be scared! Just be prepared to work your little witchy butt off learning, because if a Crone is trying to get your attention, you have some learning to do. (I will be doing a segment, or possibly a few on Hecate in the future. FYI, she is NOT a crone Goddess, contrary to popular belief.)
Will (insert your Matron/Patron here) always be my Matron/Patron?
Not necessarily. Many people "go through" several in their lifetime. It isn't a bad thing for your Matron/Patron to stay the same or change many times. Now, if you say you have a new one every day or week, then they probably weren't your Matron/Patron to begin with. From my experience, they tend to stick with you until they've taught you all they wanted to, then they'll quietly slip out to make room for the next one. I know that sounds weird lol, but that's been my experience. They don't just up and abandon you (unless you REALLY piss him or her off, then s/he might). What has happened with me is once I started mastering something that I had begun learning after a new deity came to me, it was like that deity sort of stood off in the corner observing for a while, then once they were satisfied that I had it, their presence slowly became less evident. If you've taken a Psychology class, it's sort of like scaffolding.
So what do I do after I figure out who my Matron/Patron is?
Whatever you normally do, pretty much. You can invoke him or her during rituals (instead of just calling on the God and Goddess, or the Lord and Lady, etc.), if it's a she, like Selene, then you can Draw Down the Moon calling for her specific energy, instead of just lunar energy. (Before someone jumps my shit over this, I am fully aware that there are many ways to DDM, honor deities, etc., I don't need you to tell me.) You can devote your altar (or one of them, if you have several) to him and/or her, you can dedicate your BOS to him or her, you can do daily devotions to him or her, etc. The possibilities are pretty much endless. You just need to do what feels right for YOU.