Welcome to the forum
To start with, if I'm understanding correctly, the Aus visa is not like the US one where they give you a long (5 years for me) visa, with a 6 month restriction of continous time, its a 1 time thing that has to be renewed every now and then? Would this mean if I went to visit her in the future again, I would have to apply and pay the fees again?
So you can be granted a 3, 6 or 12 month visa which could be single or multiple entry. Whatever conditions they attach to your visa you would have to abide by (so for single entry, you would only be able to enter once and stay up to the maximum time etc)
Also, they way I wanted to visit her was to stay in a hotel in the town where she lives for a couple days, then take her for some sightseeing together to Sydney or maybe Canberra (since I've never been, I really want to visit this cities!) Which stream do I apply for? I think the "Tourism" one most closely resembles what I want to do, but would I still have to say I'm going to visit my girlfriend? I've seen letters of invitation online, would my gf have to write one of this for me (even if I will NEVER stay in her house and will be paying for my accomodation, if anything, she will be staying with me in hotels and such.)
If you're coming for a visit then the tourist stream would seem best. There is a family sponsored stream but from my understanding, that would not apply to you (unless you have family here).
It might be a good idea for your girlfriend to write you an invitation letter but I would check with a registered migration agent first.
Moving on to the documents, it tells me to attach a "Family Composition", but my country does not issue a family book of any sort. Our civil registry probably keeps track of who is who's descendant, but they don't release any sort of official document to support this. What do I do?
Do you mean this: http://www.border.gov.au/forms/Documents/54.pdf
? (scroll down, page 2 is in English).
For "Planned tourism activities, Evidence of", how would I show this? I'm waiting on the visa approval (when I apply) to buy tickets and such, how can I even show where I'll be staying if I haven't purchased anything yet? Like I said, I'll probably visit Sydney and Canberra with the GF, but I'm not sure how to show this without actually buying stuff.
You could give an itinerary of what your plans are but I would again check with an RMA about this.
Finally, for "Financial Capacity - Personal, Evidence of" how much would I have to show to have in my bank account? I'm planning on staying for about a month.
If everything is pre-booked (accommodation, flights etc) then around AUD$500/week is the figure I've seen people mention so in your case it would be around AUD$2000 (or at least show that you can access roughly that amount, credit cards etc).
I believe that Panama is classed as a high risk country to Australian immigration (since passport holders from the country don't qualify for ETA) so it might be worth submitting the visa application through a registered migration agent, they will know what you need to maximise the chances of successfully having the application approved
I hope this answers some of your questions