Experience is the measure of a character's ability in his or her chosen profession, the character's class.
Each PC begins the campaign at 1st level with no experience points accumulated. <note exceptions>
Thereafter, as he or she completes adventures and returns to an established base of operations,
the DM will aword experience points to the character for treasure gained and opponents captured or slain and for
solving or overcoming problems through professional means.
Characters with high scores in their major characteristic ability area might be entitled to an experience points bonus
(see CHARACTER ABILITIES).
When a sufficient number of experience points have been gained,
the character will gain an experience level (see CHARACTER CLASSES).
It is important to keep in mind that most humans and demi-humans are "0 level". <note that some 0-level characters can advance to level 1 and beyond>
They do not have the ability to gain experience levels.
Player characters are unusual and superior.

Gaining XP through the acquisition of gp and by slaying monsters might be questioned by some individuals as nonrepresentative of how an actual character would become more able in his
or her class. Admittedly, this is so, if the existence of spell casting clerics,
druids, magic-users, and illusionists is (unrealistically) granted; likewise,
dwarven superheroes, paladins, elven thieves, half-orc assassins, and the
like might gain real experience from altogether different sorts of activities.
This is a game, however, a fantasy game, and suspension of disbelief is
required. If one can accept the existence of 12' tall giants, why not the
rewarding of experience points for treasure gained? While praying and
religious-oriented acts are more properly the activities for which a cleric
would gain experience points, this is not the stuff of exciting swords &
sorcery adventure. So too, fighters need physical training and weapons
practice, magic-users long hours of study in tomes of arcane lore, and
thieves the repetition of their manual skills and discernitory prowess; but
none of this is suitable to gaming. It is, therefore, discarded and subsumed
as taking place on a character's "off hours".

As a rule, one point of XP will be awarded for one GP
gained by a character, with copper pieces, silver pieces, electrum pieces,
platinum pieces, gems, jewelry, and like treasure being converted to a
gold piece value. Magic items gained and retained have only a low
experience point value, for they benefit the character through their use.
Magic items gained and sold immediately are treated as gold pieces, the
selling price bringing an award in experience on the stated one for one
basis.

XP awarded for treasure gained -- monetary or
magical -- are modified downward if the guardian of the treasure
(whether a monster, device, or obstacle, such as a secret door or maze)
was generally weaker than the character who overcame it. A 4th level
character versus a single orc is an overmatch, and only about 10% of the
treasure value gained could count towards experience points; but if nine
or ten orcs were involved, the experience points awarded would generally
be on the one for one basis.

Monsters captured or slain always bring a full XP award.
Captured monsters ransomed or sold bring a gold piece: experience point
ratio award. Monsters slain gain a set point award. Low hit point/dice
monsters have a low XP amount. Monsters with high hit
point/dice have large XP awards. Special abilities such as
magic resistance, spell capability, gaze or breath weapons, regeneration,
and the like also increase experience points amounts.

Finally, clerics' major aims are to use their spell abilities to aid during ony
given encounter, fighters aim to engage in combat, magic-users aim to
cast spells, thieves aim to make gain by stealth, and monks aim to use their
unusual talents to come to successful ends. If characters gain treasure by
pursuit of their major aims, then they are generally entitled to a full share
of earned experience points aworded by the DM.

Your DM will award your character(s) XP as explained.
He she has detailed information respecting this subject, and a chart of
experience points to be given for monsters slain, with bonus points shown
for special abilities of monsters. After being awarded points, you will be
expected to add any bonus due for high major characteristic ability, total
the whole, and record the number of experience points now possessed by
the character. Remember, character henchmen will gain only one-half of
total experience. Your referee might require you to inform him or her of
new experience point totals in order to keep records. This prevents any
"fudging" on the part of over-zealous players.