My main character is a plain fighter, with a constitution of 18, str of 18 etc.
But since the beginning of the game Minsc himself had about the same amount of HP as me. Since when do rangers have the same hp as fighters?
Also something else...I wondered if our thac0 and number of attacks would difference much so I ran a test.
With a CLUA command I put my fighter's and Minsc's exp to 2950000 exp.
The result: Minsc who I have equipped with a double handed sword has 5/2 attacks (with 2 or 3 prof slots into double handed sword) and my fighter, wielding 2 long swords and 5 prof. slots in longsword, and three in dual wield, has only 4 attacks.
And Valygar wielding two katanas had 7/2 attacks.

Another thing, when I started the game with a paladin as main character, he has less hp then Imoen (while the paladin had a con of 16).

I would like to know your opinions about this, since it doesn't seem to make sense with the ad&d rules.

I'm not too familiar with AD&D rules other that what is detailed in the BG2 manual however in response to your queries:

Minsc himself had about the same amount of HP as me. Since when do rangers have the same hp as fighters?

Fighters and Rangers are subclasses of the Warrior class which uses 1d10 plus constitution bonuses to determine HP and both receive 3 hp per level after 9th level so that should explain why Minsc and your hps are similar.

With a CLUA command I put my fighter's and Minsc's exp to 2950000 exp.
The result: Minsc who I have equipped with a double handed sword has 5/2 attacks (with 2 or 3 prof slots into double handed sword) and my fighter, wielding 2 long swords and 5 prof. slots in longsword, and three in dual wield, has only 4 attacks.
And Valygar wielding two katanas had 7/2 attacks

This would mean that:

Minsc using 2H Swd - 2.5 attacks per round
fighter using 2 longswords dual wielding - 4 attacks per round
Valygar using 2 katanas dual wielding - 3.5 attacks per round

Valygar can't achieve grand mastery with his weapons so that explains how he has less attacks/rd than you. However i believe Grand Mastery was not properly implemented in BG2 which would explain why you *only* have 4 attacks per round. Someone else might want to confirm or deny this. Believe there is a rules fix, which you can download here.

when I started the game with a paladin as main character, he has less hp then Imoen (while the paladin had a con of 16).

Remember that Imoen had 7 levels as a thief, so assuming she maxed out her hp each level with con 16 she would add have 7d6 plus 2hp/level for a total of at least 56. Then to reach level 8 as a mage she would have added at least 6 hp for a total of 62.
Your paladin assuming you started the game at 7th level (89,000 xp i think) you would have 7d10 plus 2hp/level for a max of 84. Hmm. That's a bit strange. HAve you got max hp increases set?