early game you are not a good fighter until you get level 6 and your ultimate. You should almost always be top lane solo, before getting level six sit back and harass or last hit with cleavers, when the enemy tries to attack you, either back up or make sure there are no minions between you two, then fling a cleaver and pop your ad, start beating them up, then back off NEVER GET GREEDY, NEVER TOWER DIVE BEFORE 6. Make sure to keep note of who has ignite on the enemy team, in team fights it is okay to pop your ult around 80% hp if you see yourself taking damage, DON'T WAIT TOO LONG TO ULT, THEN YOU GET IGNITED AND BURSTED DOWN. In lane use it around 50% in order to be safe and not make them dive you if you wait too long.

You are the tank of your team, it is your job to initiate, BE SMART WHEN YOU START FIGHTS. Make sure to catch people out of position, you have a ranged slow to catch him, high move speed, and then the frozen mallet will seal the deal once you get to him, that is an easy kill for you team. Do not be afraid to take damage for your team, with your defense and items, you should able to take a huge beating, and then you use your ultimate and you're very high hp again. You need to threaten people in fights, if it is diving through the enemy team to attack their carry or support, or slowing down an enemy to nothing while they are trying to attack someone important on your team.

I like to have pure ad runes to help with early game damage, his is quite bad if you can't hit every cleaver, because I like to rank masochism quite a bit, the ad runes on top of that attack damage can really devastate your opponent. The armor and magic resist just help you live and take less harass. Some may ask "Why not hp quints?" well because without magic resist and armor your health is a lot easier damage, and makes health less effective, also makes your ultimate less effective.
VERY IMPORTANT TO THIS BUILD, DO NOT NEGATIVELY CRITICIZE UNTIL YOU TRY IT OUT FULLY.

You can choose to put more in defense if you'd like, this build gives quite a good amount of defense and some offense, as well as helping you farm and stay in lane as long as possible till you hit level 6. One common change would be switching the life steal points to the increased move speed while over 70% health. I personally detest any utility points for mundo, he does not need them, he is very tank and defense masteries make you un-killable, and then offense makes you a threat which causes the enemy to focus you, which makes your defense runes and masteires to help even more!

Items give you good utility, defense, and a LOT (450+)of damage if you get low and pop masochism. My only comment is the last item, sometimes you may want more magic resists, or a omen, or thorn mail or my favorite, A TRINITY FORCE! trinity force gives you damage, hp, sheen effect, move speed, and critting with 450 damage cant hurt ;). I hate the item warmogs, it gives you nothing but health, no utility for your team, no resist to make you harder to kill, it is only taking up an item slot, since you're playing mundo you already have a lot of health, and health regeneration, a warmogs will only make you squishier!! and less of a target to focus, because you're not a character like amumu or galio, you need damage or a reason for people to focus you, i love frozen mallet and sunfires because of this. It allows you to have a lot of health, and damage, and good resistances! guardian angle is also a good item.

you cam choose to get one rank of burning agony at level 4, its just my personal preference to ignore it. otherwise skilling is standard for mundo.

if you want you can switch exhaust out for cleanse (to get those ignites off of you) or heal (but it is really not needed if you play smart and farm to get items fast.) make sure to exhaust important targets, usually ad carries or sometimes AP carries right before they blow their load all over your team.

Mundo is a tanky/dps, be careful early game, farm well, use your ult early on to prevent yourself from being bursted and dying, have fun! ;) PS runes are VERY important in the build.

early game be careful to not get harassed too low while going for creep kills, creeping if VERY important for mundo, you need the gold to get ahead and then you're unstoppable, once you get enough health regeneration feel free to use burning agony on the creep waves, along with a sun-fires you will be able to farm extremely fast and efficiently.

While laning make sure to not let your opponent farm to his/her hearts desire, you NEED to dominate people like irelia very early on or you well get destroyed for the rest of the game. If it is a strong character 1v1 back up and harass down with cleavers, if you think you wont take much damage from them you can fling a cleaver then walk up with masochism and start beating them up badly. If you are against a skill shot dependent champion try to stay right behind your melee minions, if they are weaker than you, patrol their caster minions and fling cleavers at them constantly to keep them from farming and even getting xp.