Hi rkbmx,
Welcome!
I always think it’s great when people come from a different background into Yoga- I think it helps in dispelling the myth that we’re all floozy types who sit around all day chanting and occasionally pull ourselves into contortionist postures 
As for giving you some guidance into practice, the first thing I’ll say is to find yourself a good class! Learning at home is good, but it has it’s limitations. In a class not only do you have a tutor to teach you the postures correctly and adjust you if you need it, but they can also help you identify your physiological strengths and weaknesses and suggest suitable postures to develop felexibility, strength etc. There’s also the opportunity to learn from others (especially in partner Yoga, where you use someone elses’ body to facilitate your own posture) and also ask questions. So, if you can, pleasedo get to a class, you won’t regret the investment!
However, in addition to attending a class, personal practice is also really important and you can learn a lot. I always maintain that the greatest teachers of Yoga are the postures themselves, and they communicate to you what you’re good at and where you’re flexible, and what needs working on. You highlight lower back, core strength and ankles…great news! nearly every posture works those things!! A resource you might find helpful is Yoga Journal. If you go to the toolbar at the top, you should see a button marked ‘poses’. Click on there and you get taken to an index with a list of common postures. You can also do a search where you can submit an anatomical focus (eg ankles) and it will come up with potures that will work that part of the body. I ran a search for you, see here:
Asanas for Ankles
Asanas for the Abdomen
Asanas for the Lower Back
Click on any of the postures to bring up a photograph and description of how to get into it.
Hope that helps!