Hello Mole.
It sounds brilliant! I love a good project to get stuck into :)
I'd be tempted not to do too much initially, and just see what comes up. You'll get all kinds of surprises next spring, there may be flower bulbs tucked away, and self-seeded flowers, and flowering shrubs - it's an exciting surprise garden! If you weed-killer it all, you won't have any of that.
It's unlikely that there will be anything useful lurking in the raised beds, so you could probably weed those and dig in some manure, then it'll be ready for planting in spring.
When you say it's overgrown... do you have brambles? Keep them and collect the blackberries, they'll be fruiting before long, if they aren't already.
If you really don't have much knowledge, I would get rid of anything that you know to be weeds (like nettles, dandelions etc) but anything you're not sure of, leave it until you can figure out what it is. You might have all kinds of fruit bushes hidden behind the weeds, or anything!
When I moved into a rented place and sorted out the garden that had been left to it's own devices, I found a japanese maple (which I love) and a rabbit hutch that even the owner didn't know was there (no rabbit, thankfully).
Keep us posted, I'm all excited for you!