The Monk is one of the harder classes to master, but one of the most powerful when you do.
Beginner Level Tactics
The Monk's penalties add up to him doing about half as much damage as normal. That's bad. But, his resistance means that he takes half as much damage as normal. As long as you are fighting non-magical monsters, this is a wash. If you are playing a Human or Orc, you can increase your damage through conversions, tilting the math heavily into your favor. For most classes, getting a+50% bonus to your damage is just that, a 50% boost. But because the Monk starts at -30%, he benefits proportionately more. A +50% bonus is more like a 70% increase in real damage. He benefits even more from base damage boosts, like the Fine Sword or Orc conversions. So as long as you're fighting physical monsters, you should be able to get a significant advantage at little cost.
Magical monsters are going to screw you at this level of play. Sorry about that. Keep the Fireball glyph around because it makes fighting Warlocks and Wraiths easier. Dragons are always going to be a problem.
Intermediate Level Tactics
The Monk's doubled healing is a really powerful ability, but you have to leverage it. If you are just playing like normal, and only using it to passively refill your health after killing a monster, you're going to find it to be underwhelming. The Monk needs to use his healing while fighting to really excel. This allows him to consistently fight higher level monsters, as long as exploration space is available. The people above me int his thread have already talked about how that works, so I won't repeat it here.
Advanced Level Tactics
Once you have a higher level of general game knowledge, you'll want to look into resistance stacking. The most resistance you have, the more valuable each point of added resistance is, and the Monk starts with a whopping 50%. The simple Tower Shield (+10% resistance) gives the Monk the same proportionate advantage that a +20% effect would give someone else. You' want to stack this as high as it can go, up to the maximum of 75% if the opportunity arises. If you are stacking up bonus damage AND bonus resistance, the math because hilariously titled in your favor and the Monk becomes a beast. Common ways of doing this:
Monk of Taurog: Prep or find Taurog. Get the sword (+5 damage is excellent, and is especially good for Monks), and get the Armor (you'll have 65% resistance after that). The Helm makes magical monsters slightly less painful, but it's not transformative. The Shield isn't very good for Monks because of the order of operations (reduction is applied before resistance, which is bad for high resistance characters), but you'll probably want it for the passive 5% damage bonus. You'll be able to beat most maps Warmonger with this.
Monk of Dracul: Find Dracul (prepping is not recommended) and get Blood Shield. That's it, really. Blood Tithe is as good as always, and Blood Swell is great too. Blood Curse actually helps your regeneration and is worth getting if you're confident that you won't be hitting level 10. Blood Hunger works antagonistically with your resistances and is not generally recommended. Life steal Monks are very good, but require more specific planning (you need to have the Vampiric Blade and a single dose of Blood Hunger at the same time to make it work, and in doing so, you are giving up on extreme resistance stacking).
Monk of Binlor: Prep Binlro and start amassing piety. Spam the Stoneskin boon at every opportunity. Stoneskin temporarily maxes your physical resistance (75% resistance !!!) and gives you a permanent 3% towards magic resistance. If you really work it, you can end up with a Monk with 75% innate resistance to magic. PISORF is also really good for Orc Monks, but that's a whole other topic.
There are other strategies (caster Monks, Codex Monks, etc), but I consider these three to be the most straightforward ones, playing naturally to your strengths.