There are three different typs of heroes to choose from: carry, tank, and support. Each hero has its own unique ability and role to play on each five man team. For example carry's are your strongest asset when it comes to battles. They have the highest damage per second and get the most kills for your team. Tank's are heroes that have the most health. They too are good for team battles, but their main worth is pushing lanes and destroying towers. When it comes to getting to the enemy base the fastest, tank's are your best option. Support heroes are good for healing your team, slowing or stunning the other team, or raising certain stats of the other heroes on your team. They may seem very weak or worthless, but just like carry type heroes they too can be a great asset in battles. Now to choose a hero I recommend doing a few bot matches, AI controlled heroes, first in order to find the type and hero you feel the most comfortable playing. The bot matches are also good for getting to know your heroes build and abilities.
Hero builds refers to the items you get for your hero from early game items to the strong late game items you get from farming different enemy bots and buying them from the shop. The various items gives your hero different stat boosts and abilities such as the shadow blade which raises movement speed, but also gives your hero the ability to go invisible. Depending on what you build on to your character makes the difference of if your hero will be strong late game or no help at all to your team. Also hero builds can even make your character in to a different type such as building enough items on to a support to make him strong enough to be a carry. Through hero builds this is possible if you familiarize yourself enough with the items and the correct combination of items. In the end it is always a great choice to play your role in a game, but taking the game over by yourself is always an option too!