Train your fighting skills is easy, you need to do is to attack the enemy. Depending on how much damage you deal, you will get a certain amount of XP. Now, if you ask a more specific question, such as "how do we train the strength skill in Runes of Magic", you would open up this menu, the attack style menu:
Different on the menu button to customize your character(Runescape powerleveling) attacks. According to the choice of attack, your character will use a different attack (accurate style, aggressive style, and so on), use a different type of attack (stab, slash, crush, and so on), and get XP in a different skill. Using the previously mentioned question, "how do we train the strength skill", you would select the "aggressive" style. It's pretty self-explanatory. Here's a chart for information on the different attack styles, what they train, and how much XP you will get.
These aren't really important to know but it might be handy to know these, granted you know what they mean. Remember, math isn't a useless skill that you will never need. Pay attention in class(runes of magic gold). Looking at the table, you will see that beside each attack style, it says what it trains. This is pretty self-explanatory, IE a "defensive" attack style will train defence XP, and an accurate trains attack. To end the question that many people wonder, "how do you get constitution XP", you get it by dealing damage to an opponent. The table there pretty much says all you need to know about the attack styles.