What should i know before starting BG2?

Technically i have already started it but suggestions are welcome :)

I'd say don't get put off by the apparent linearity of the opening dungeon. I personally really like it, but I know some people don't. It lasts around 3-4 hours and after that the game opens up very quickly. Also, Breach is one of the most critically important mage spells at mid-high levels - don't leave home without it (or your trusty space hamster!).

Otherwise, Id say the less you know, the better. Enjoy the ride! :-)

Btw, what class is your main character?

I believe it's called ranger or something
4 star longbow proficiency. Imported from BG1

That's a sub-class of Fighter, although I believe it has its own stronghold. I'd say if you haven't figured out where the Ranger stronghold is by the time you get to Chapter 3, it might be worth looking it up on the wiki, to make sure you don't miss out on it. BG2 has 9 'strongholds', which are locations the main character can adopt to become like a base of operations. The stronghold you can use depends on the class of your main character and you get access to them them by completing certain quests. They each have their own quests and story line, which add another layer of fun to the game and a lot of replay value (as you only have access to one of the strongholds on each playthrough).

^Ranger is its own class (and has its own sub-classes - Archer, Stalker, and Beastmaster).
Archer is awesome - had loads of fun playing as one a while back.

And yes, the Ranger stronghold is different from the Fighter one.

Yes, of course it is, sorry. It's been a while since I played these games. I was confusing it with the Archer sub-class of Ranger.

Watch out for the timed quests.

Also: the Silver Pantaloon is given in an evil quest (ransom for hostage). You can not do that quest if you have Keldron in the party.

I'm on a playthrough right now so it's fresh in my mind, lol.
(Playing a Fighter>Thief dual-class this time though (Assassin kit on my thief as it's fun to poison mages)).

Be careful while playing the Archer class in BG2. Archers are amazing in BG1 and overpowered in IWD. However, "I believe", they will have serious problems in BG2. I never played as ranger myself, but I always have Minsc, who is a Ranger. In BG1, Minsc finishes the game with around 40% kill ratio while he is about 5% in BG2. I recall arrows being useless often and made him either dual wield maces or use 2H swords. His low intelligence does cause a problem in the Underdark tho.

Hold on to your magic arrows and seek out Bows that auto does +x damage for monsters that are immune to normal damage. Someone with better experience may provide better information, or ways to make ARCHERs effective in BG2.

While Archer is overpowered in BG1 and loses that advantage towards the end of BG2, it's still a decent class (not that I'd try soloing with one but with a party, it's fine). Not the most powerful but still useful. Arrows come in useful varieties (keep some fire/acid arrows on-hand for finishing off trolls, arrows of dispelling for mages (also arrows of biting to poison them), arrows of detonation for going 'boom' and arrows of piercing for damage). Sadly, not all arrows are readily available, but I had no trouble finishing to the end of TOB with my Archer (albeit in a team that also had a mage, thief, cleric, and fighter). Having a magic bow like the Bow of Gesen helps with the ammo problem.
Most importantly - is it fun to play for you? For me, it was :)

Useful list of bows and where to get them (Spoilers, obviously, in terms of bow and ammo locations)
http://mikesrpgcenter.com/bgate2/weapons/bows.html