Hi Trust, I attended the 2014, 2015 and 2016 Belgian GP's with a GA weekend ticket, so I think I can help with some of your questions.
Use Friday to explore the track and decide which spot you want to sit for Saturday and Sunday. For all 3 years I sat at Pouhon on the Saturday and Sunday because of it's great view and access to giant screen TV's. Kemmel is huge with constant banking along it with 2 giant TV's and is popular. The Bus Stop also has room for spectators with a giant TV but is very cramped. Last year there were also 2 small temporary grandstands between the exit of Pouhon and the entry to Fagnes that were free for GA spectators to use, at least on the Friday. Just make sure you've decided on a location before you turn up on the Saturday and particularly the Sunday so that you can head straight there and claim yourself a good spot.
On the Thursday you can only walk on the start-finish straight, a little bit of the exit of La Source and the pitlane. On the Sunday you can walk most of the track after the race. In 2014 and 2015 I accessed the track at Eau Rouge but you can clamber under the gates at the bottom of Pouhon if you don't mind getting your hands dirty!
In terms of autographs, the best thing that you can do is arrive as early as you can at the La Source gates and run as fast as you can when they open at approximately 4pm. In 2014 I got Kimi's and Fernando's autograph and in 2015 I got Kimi's autograph again. However, in 2016 they changed the process. In 2014 and 2015, the drivers came outside of their garages but in 2016 they had to go one by one to a "pen" at the exit of the pitlane. It meant that a small group of fans got every drivers signature whilst the vast majority could barely even get a view of the drivers. It was a really stupid way to organise it. Either way, my advice would be to go to the garages because at the very least you get 2 hours of watching pit stop practices and cars being slowly rolled to the scrutineers bay. You will also spot a lot of famous engineers and media personnel.
To end on a sour note, the organisation of the Belgian GP is terrible and it got worse every year I've went. The massive influx of Verstappen fans have made the weekend overcrowded but this is only exacerbated by the incompetent police, security and stewards. Last year they had the genius idea of opening only one gate at the Les Combes entrance and having a handful of security staff checking each spectator's bags. This caused a dangerous bottleneck for the crowds entering the circuit and I am really surprised no one got hurt. Because of this I am not attending this year's GP. My advice to anyone going is to arrive early and be prepared for over 6 hours of queuing on Sunday afternoon. Unless you've got to get back home ASAP, stay in the track as long as you can, until you get kicked out.
Despite my last paragraph, I can guarantee that you are going to have a great time Trust. Not only are you going to see a sport that you've loved for years for the fist time but you are also visiting a circuit where every square cm of track contains history and memories. Enjoy it mate!