There are 72 heroes to choose from, 36 for each side. Each has unique abilities, often completely recoded by the creators. There are three attributes for heroes, one which will be the hero's main attribute (damage will be based on this attribute).

Strength: Increases hitpoints (19 hp per strength), minor health regeneration.

Agility: Increases attack speed (% per agility), increases armor (7 agility = 1 armor).

Intelligence: Increases mana (13 mana per intelligence), minor mana regeneration.

Note that all strength based heroes are melee ranged, most intelligence heroes are ranged, and agility based heroes come in both flavours. Generally, ranged do less damage because of their range advantage.

Now, which hero should you choose? Each hero has its own weaknesses and strengths, not just against other heroes but also with regards to the period of the game. Intelligence based heroes are spell based and are strong in the early game but lose effectiveness late game. Strength heroes are weak in the early game but strong mid to late game. Agility heroes are the weakest in the early game but the strongest late game.

My suggestions for beginning heroes for new players:

Knight Davion (Dragon Knight)	Zeus (Lord of Olympia)	Traxex (Drow Ranger)

Lucifer (Doom Bringer)	KelThuzad (Lich)	Rotundjere (Necrolyte)

I suggest these heroes either because they are easy to learn, have a passive hitpoint regeneration, or are very effective farmers. 

Pre-Game Purchases

So you've got your hero and now you are overwhelmed by the vast number of items. What do you get?

Boots of Speed 500 g +50 movement speed
While others may disagree, I do not believe Boots of Speed should be your first item, though it will be a needed item after purchasing regeneration items to allow you to stay in the lane. If you do not have this item by the mid game you will be outrun in most chase situations.

Ring of Regeneration 375 g 2 hp/sec regeneration
The regeneration item of choice for most heroes in almost any game mode. Do not be fooled by the small number; this item is effective and allows you stay in the lane to soak up experience and gold. If you have a friend in your lane, this can be upgraded to a headdress of rejuvenation. With a headdress, your teammate will receive the same regeneration, allowing him to focus on another item.

Ring of Health 875g 4hp/sec regeneration
This item is my personal favourite for melee heroes in All Random games. It has double the regeneration of the Ring of Regneration, but costs more than double. You can buy two RoR instead of one RoH, but the upgradability of a RoH is much greater as it can create a Perseverance, an item which can be found at the “secret shop”.