Dr mundo is a powerfull tank with big health regeneration rate.So getting items to make his health regeneration increase to very high ammounts will give u win.

By level 9 u should have maxxed your Q also known as Infected cleaver while putting 2 points in masochism 1 in burning agony and 1 in sadism.Put points in your R whenever you can
and max out your E by level 13 finish with leveling Burning agony.

To max out his armor i use the Greater glyphs of resilience, like said in the introduction you need to max out your health regeneration rate though putting all runes for health regeneration is exaggeration but having the Greater seal of vigor will be a great regeneration boost.Sometimes overfed mages deal very high damage to you (not always though)
so putting the Greater marks of shielding will prevent some of their damage (if u dont have any problems with them like i do u might want to replace them with Greater marks of focus to reduce your cooldowns.

I use a 9 21 build in masteries to increase the deffences of mundo and reduce the cooldowns while having summoner's wrath to improve exsaust i dont preffer summoner's resolve for some increased healing.

I use heal and exsaust for more survivability.
Summoner spells that you can use instead are:
Flash and ghost help you get out of combat when u r low on health
Ignite, to add some more damage on the enemies.
Promote its not used that much by any1 but it can be a great resource of earning money early game
Summoner spell u shouldnt get:
Smite unless u r no solo top but jungling
Surge no use for you
Crairvoyance u r not the support of this game

I use this item build to maximize my health regen while i get some armor aswell.
So start by taking a regrowth pendant and a health potion
on your first recall try to get at least the boots of speed and the cloth armor or even better the ninja tabi.On second recall start building your spirit vissage and get 2-3 potions to survive sometimes when they feed me too much or i need to stay up there too much i buy both SV and NT at the same recall.Continue with a Warmog's armor a Force of nature and then another warmog's armor.To finish u should purchace the atma's imphaler to turn 1.5% of ur health in attack damage!
Items u can get too
Sunfire cape its an awesome item that can boost ur damage while giving u more survivability aswell!
Thornmail for its armor and and its damage returning buff
Randuin's omen for its armor bonus and cd reduction.
Trinity force for damage boost.
Mercury's threads u get the same effect by your w but its a good boost of it though

Items you should avoid purchasing
Frozzen heart players will think u as a troll for purchasing mana
Frozzen mallet it might look like usefull but its actually useless cuz u r having a slow already and u might want to spend your gold somewhere else

Pros
High amounts of damage and health regeneration
Good survivability
Very good escaping skills
No resource so never runs out of anything

Cons
Uses Health to use his abilities
Not very strong CC

Not many things to say here just kill creeps with your Q early game to avoid getting high ammounts of damage by the enemies if u have the chance while getting back to your lane after death or recall u might want to kill the golems at your path or the wolves.It will be a great way to get gold faster and buy your items!

Try playing defencing until u reach level 6 and get sadism.
After u get to level 6 be more offensive but dont exaggerate.
In team fights u should be using ur cleavers to slow the focussed enemy down so they dont escape that easy remember to keep burning agony active for even more damage and ofc your ultimate if u need to be healed or if u need the speed boost it gives u!
in late game you are a killing machine with high ammounts of health regeneration armor and damage so be the most offencive u can!