how do you level up attack and how do you level up strength? whats the difference eg attack kill things strength take lots of hits?
Attack or Defense XP is gained by striking mobs or players, depending on what weapons you use. Most weapons give Attack XP but a few give Defense XP. If you strike with a non weapon, such as a pick axe, Attack XP is given.
Strength XP is not given directly for a specific action (*with 2 exceptions), but it is given by many actions as secondary XP. Striking a mob or player, mining, digging, chopping a block, etc. I don't know them all off the top of my head and I'm not on my dev computer so I can't list them all atm.
* There are 2 weapons in the game that allow you to get Strength XP directly as the primary XP, without leveling other skills.
Health XP works the same way as Strength XP but for different actions. Some actions give both Strength XP and Health XP as secondary, like Striking a Mob or player, some actions only give Strength as a secondary and some only Health as a secondary.
Secondary XP is always a (smallish) percentage of the primary XP.
The higher your Attack level is, the higher the chance you will hit your opponent when using Attack based weapons. And the higher the class of Attack based weapons you can use. Higher class weapons have higher Max Hit values (can strike more damage). Your Attack level has no bearing on how much damage you can inflict, only the chance you will hit in the first place. Your weapon and your Strength level determine how much damage you can inflict (Max Hit). So players with high Attack and low Strength will hit often but not very hard. Players with low Attack and high Strength will hit less often, but when they do, they will hit harder.
The higher your Defense level is, the higher the chance you will hit your opponent when using Defense based weapons (your Attack level is also factored in here as a smaller secondary value). And the higher the class of Defense weapons you can use. Also the higher the class of Armor you can wear, also the higher the chance your opponent will miss you when they strike. Defense does not reduce the damage you will take on a hit, it only reduces the chance you will be hit in the first place. As for Attack, your Defense level has no bearing on how much damage you can inflict.
Your Strength level is factored into the Max Hit of the weapon your using so the higher your Strength level, the more damage you can potentially do. Higher Strength also has secondary benefits such as allowing you to mine, dig, chop faster (regardless of tool), and some others.
The higher your Health level, the more Hit Points (Health Bar) you have, and therefor the more damage you can take before you die.