Hey there,
just read through this topic and I hope you don't mind me adding a little advice/personal experience to the great advice you've already been given.
Dogs destroying things at a young age is really common. I think it mostly happens when they're teething, but it can be because of boredom, or it can just be naughtiness
All our dogs have done things at some point- to run down this list:
1. First dog: German Shepherd, used to chew on the brick walls
2. First *****: Labrador x Colly, shredded a new carpet and the underlay.
3. Second *****: German Shepherd, chewed through a radiator pipe and flooded our down stairs hall. Also chewed wooden banister and skirting boards.
4. Third *****: German Shepherd X Colly, she's a rescue dog and was awfully abused and eventually was left to starve to death before she was rescued. It took us 2 and a half years to sort out her separation anxiety- she would mess every single time we left the house, even if it was just for a couple of minutes. She also still occasionally steals food.
5. Second dog: German Shepherd. Codie is about as old as your girl and is another rescue, and you've been rather lucky so far by the sounds of it!
Codie has destroyed a grand total of: 1 internet router, 2 house phones, several pairs of shoes, several pairs of socks and underwear, the spare bedroom mattress, new wall paper in several places, chewed my parents door, shredded the bottom of their curtains, my mums camera printer. The list goes on.
Codie was dumped in a pound and was due to be put down on Christmas eve. We now believe that this was down to how destructive he can be (he seems to be growing out of it now, touch wood), and it's just something you've got to bare with. Take your lass for as many walks a day as you can, tire her out as much as possible and this should reduce the amount of damage she causes if it's boredom that's causing it.
As to training, the best way to do this is make it as fun as possible for her. And repeat everything. And make sure you enjoy it too
! I know your 15, but training has always been a favourite part of keeping dogs of mine. At 12 years old, I trained my lab X colly to sit, stay, heel on lead, heel off lead, close the door, jump through hoops, high five, roll over and close the door. Anyone can do it, you just need patience
Start off by training the essentials: sit, stay, leave/give, and very, very importantly come. Find out what her favourite treat is, and use that to bribe her into doing what you want. Once she does it every time for a treat, start getting her to do and with hold food and just give her praise instead. Dogs usually are really willing to learn and love to please their humans because of the amount of fuss they get when they do something right
As to spaying and neutering, I'm not going to add much more on to this as everything has been pretty much covered and it's obviously a topic that people (including myself) are very passionate about. I only want to add that not spaying your ***** can actually result in her getting cancer of her womb, mammary cancer or an infected womb. This is much more common than you realise and it's a real danger, and your dog doesn't even have to be old for this to happen. You're likely to loose your dog if she contracts any of these. Having her spayed will significantly reduce the risk of her getting mammary cancer and will totally prevent her from getting cancer of the womb/infected womb. That's all I'm going to say
I hope this crazy long post helps you in some way or gives you some ideas
Good luck with your girl- everyone deserves a dog as a companion, you just need the patience and devotion