English is not my main language, so please forgive my errors.

I'm 26 male and I want to know wich are the main things a gentleman
should do for his loved one...

I mean hold the door, hold chair when she sits... 

I remember that my mother told me when I was younger that a man should
always walk on the right side of his loved one. Why (she didn't knew
herself) and where this come from?

I already hold the door & things like that, but I must miss things
thus the origin of this question :)

I can't speak for all women, but frankly, all the old-fashioned
etiquette doesn't mean very much to me. I have had boyfriends who made
a show of opening doors and holding chairs, but who dealt out
disrespect in subtle ways. My husband isn't a door-opener or a
chair-holder most of the time, but he treats me with kindness and
dignity, as an equal. That, to me, is worth much more than a passel of
traditions. ;-)

I know, but it's also for my knowledge :) It's always good to know...

This may be of interest:

http://www.jkaugust.com/whatsnew/ritchimage-12212001.htm

Some things are just common sense, however, most of what is believe to
be good ettiquete is based on very old traditions. Because of that you
should keep in mind that traditions are by nature social, for
instance; in the west it is generally believe to be bad manners simply
burp specially if at the dinner table, in some eastern countries the
opposite is true, a loud burp after a satisfying meal is considered a
sign of good manners and a compliment to your host/ess culinary
abilities.

That said, the tradition that you speak of "a man should always walk
on the right side of his loved one" is incorrect, in fact doing so
could very well mean the exacto opposite of what you intend to convey,
good manners. The tradition had it's roots in ancient Rome, the city
was cramped (by anybody's standards) and for the most part it lacked a
working sewage system (common folks not the upper classes), because of
this people had the habbit of simply throwing the dirty used water on
out the window of the 3rd, 4th or whatever floor, it was up to the
pedestrian down below to keep an eye out for such events and get out
of the way if they saw it comming, if they did not a dirty water bath
would ensue. Often this waste water was not just water from washing
the dishes but human waste and it could contain solids as well. So, if
you were walking on the inner side of the sidewalk you were less
likely to get splashed by the sewer water comming from above, and as a
gesture of manly courtesy the males would walk on the OUTER side of
the sidewalk (not on the right of the lady), in this case if somebody
was to get an impromptu bath it would be him not her.

Not sure if this was a language barrier but you don't 'hold the chair'
you are supposed to 'pull the chair out, then as she sits you push it
in under her'. Again this is an old tradition, not quite as old as the
previous one... centuries ago women wore extremely complex dresses,
sitting down while wearing one of them was a rather difficult task, so
the man would pull out the chair and push it under the lady moving the
dress aside. Although the need for this is no longer there the
tradition persists.

Some things that are just as old and they may still be useful; 

The man carries two handkerchiefs, one for himself that he is supposed
to keep in his back pocket and one (New and Clean) for the lady, that
he is supposed to keep in his jacket, on the inner pocket.

The man is supposed to pay for the meal if it is a 'date', although
this may not sit well with all females nowadays. One thing one should
not do is expect the woman to pay (at least not until after the deed
is done... lol just kidding)

This is very old and if done properly it can be seen as a courtesy,
specially if the woman knows about it. However, it is seldom done. A
man is supposed to 'stand up' when a woman enter a room or take his
'hat off' when he first encounters her. Most people don't wear hats
anymore, and those that do would have no idea of why would they have
to do a stupid thing like that for...

The list could go on and on but most of what is good etiquete nowadays
is simply courtesy. Some of the things we do in our everyday life most
people don't know or care to know why they do it, for instance almost
everybody shakes hands but few know why they do. During the dark ages
it was a common thing to carry a weapon of some sort, ussually a
dagger. When meeting another man in a sign of good will that meant 'I
come in pease and mean you no harm' he would extend his open unarmed
hand to his potential victim (being as most people are right handed it
was expected that you would extend your right hand). The other man
would come closer do the same thing and grab his hand, neither could
very well kill the other while they were both holding their opponents
most dexterous hand. And that is how the hand shake got his start. In
the east they never had those problems, so they developed the 'bow' as
a sign of respect, originally reserved for nobility but later afforded
to everybody.

Anyways...

There's some useful advice here (scroll down to "the basics of chivalry"):

http://www.askmen.com/money/successful/41b_success.html

I agree with all of that, though I learned that in Europe, men walk on
the left side of the lady to avoid his sword banging between them   -
an also practical but much later explanation.  The "place of honor" is
to the right, now appropriate etiquette in Europe when a senior and
junior of either gender are walking together.
In the States, it is considered correct for the man to walk on the curb side.

Yes, a man should open the door for a lady, but again, in Europe, he
should preceed her into a restaurant or bar so that she not appear to
be unaccompanied.  In elevators, he should remove his hat and women
should be allowed to enter first and leave first, if this can be
allowed conveniently.

The trick these days is to manage the gentlemanly arts without being
intrusive, just being in the right position to do them  - or not do
them if the lady "beats you to the door", for example.
Bringing flowers to a hostess always goes well, roses for your beloved
 - but not for her mother or others (there is a whole code on the
meaning of flowers: rosemary for remembrance, etc.).  In America, this
may be uncommon now, but she will all be the more appreciative, and as
a foreigner one can be a little old-fashionedly formal.

How interesting... these two contradict each other... :-) what's a man
supposed to do? lol

"a man should open the door for a lady, but again, in Europe, he
should preceed her into a restaurant or bar so that she not appear to
be unaccompanied." 

This kinda makes ya wonder... are there any similar 'rules' for women?

Eli, 
He goes in first and holds the door for her to enter, which also
allows him to assure himself that it is an appropriate place to bring
her.  Maybe I should have mentioned that before.

Yeah, those two handkerchiefs:  Good idea, not just at weddings and funerals.
I learned my lesson when a jr. high teacher suggested that I offer a
girl a handkerchief, but I knew that snotty wad in my pocket wasn't
the right thing.

Cheers, Myoarin