I've been a Longbow/Shortbow Ranger with 2 bears (Brown and Black) since BWE2 and you can have great success that way. I do agree though, it is well worth the time and it's fun, to actually try out all the different weapons to see what they offer. Keep in mind something like 1h Axe / War Horn still allows you to play ranged.
ANet has done a really good job at encouraging you to play and learn. Try different trait trees, the cost of resetting your points is just the cost of buying another manual (10 silver for you first 25? trait points). You more than likely will never go wrong (ie make your character unplayable) at worse you are going to end up with a build that doesn't suit your playstyle.
- Long bow vs. short bow?
Longbow gets a bonus from you staying at range, something closes in on you swap to short bow if you really want to only use bows.
- Is it necessary to have an alternative weapon switch like a sword? Or can I easily play with a bow only?
See above, you can absolutely do 2 bows.
- I can have two pets, did I understand that correctly? The bear seems to be highly praised, but what should be my second pet?
Everytime you find a "Juvenile" pet out in the world you can walk up and hit F to tame it. Collect them all! From that set of pets you have you can "equip" 2 at a time and use F4 to swap out between them. Black, Brown, Polar (bears) Murrelow, Arctodus all have the highest Vitality (health) pools and work well as tanks each has a unique F2 ability so not all tanks are the same, if you want to focus less on pet swapping they will have the longest survival in most cases and are amphibious.
- Isn't there a simple pre-made build for a total beginner GW2 and ranger player? I mean, I don't want to focus on switching skills and using complex tactics too much, else my head is gonna explode. I need something plain and simple - shoot 'n' run lol

(and enjoy of course!)
There really is no right or wrong build just pick a primary tree that looks good and put at least 20 points into which gets you access to the Master level skill. If you really like it go a full 30.