If you play DoD the first time, I would recomment to be part of the human team. The closeness to your towers and other allied buildings and heros provides a bit safety for unexperienced players.
First of all, you have to chose a hero in one of the taverns. In those, heros are categorized by their main attribute - strength, agility and intelligence. A quite good beginner hero is Oak. This unit's spells aren't hard to use. Oak is very useful for teamplay, he is hard to kill doesn't need a complex itembuild regardless of the hostile heros.
When chosen a hero, it will be placed near your outpost. The outpost is a building where you can hire or upgrade units. If every player has chosen (or randomed) a hero, you will gain 500 start gold. Enough for 2 workers, you can hire in the outpost. Send you workers to the goldmine and move your hero to the crossing where you troops spawn. The best is to chose the crossing being right to your outpost, when looking to the center of the map.
When ever you can affort it, hire new workers and send them mining (There is a limit of 5 workers for every player).
Human and orc troops will meet at every crossing, fighting there until every hostile unit has fallen and if so move on to the next target. Main target for human troops is the lord remining in the orcish outpost you will arrive if you turn right when passing a crossing. Main target for orcs is always the human castle in the middle of the map.
When playing human, do not leave the crossing - until it is necessary to avoid death - for the next 5-7 levels. It is important to get some experience and become stronger. With level 6 or 7, follow your troops - in case they manage to pass the crossing - and support them with fighting the towers guarding the orcish outpost.
Now you should already got some gold, damn sure enough to purchase at least boots. There are ogre boots, leather boots and fur-made slippers. Playing Oak, ogre boots will fit best, like they will do for most strenght heros. If there is some gold left, powergloves are a good starter item too. If you want to chance the main attribute gain for the gloves, just left-click them, but strenght is always good to increase your health pool.
Now it's getting time to act with your allies. Meet somewhere and try to kill at least one orcish lord or some orcish heros. In case you play orc, try to get some towers. Towers mean gold for your whole ally and map control, that is important for assasinating hostile heros.
If towers you own are under attack, you can make them invulnerable for a short time by using the divine shield. Do not waste this spell, 'cause it has high cooldown. If you play orc, your lord has an akin spell but if used, it will not be able to attack for the spell's duration. It gives some seconds to move there and defend the lord.
When having problems to kill towers or lords, try to hire some catapults or rather dragonhawks. Those units are perfect support for any pushes. You could also purchase a baby dragon at the supply merchant. They aren't cheap, but very powerful.
Now you are familiar with the basics, you shouldn't have any problems mastering this game. Otherwise, feel free to ask for help, using this forum.