Play with bots (trust me) until you understand using the shop quickly, the map, lane fighting and a couple of heroes. If you want a great hero to learn early I'd always tell people to try Lich because of his great lane control (which will set your team up for a great mid game) and his ultimate is cracking for teamfights - unless you have creeps around at least.
Aim of the game is to get their centre base item dead, it's protected by two big towers (aka tier 4 towers).
Anything with a stun will be good if you're just starting out as everyones overall gold is likely to be lower than a mid-to-high skill games so CC/abilities will be more pronounced than items that can mitigate or influence the game. Vengeful spirit (aka veng) is a good example of this type of hero (an agility support although you can carry with her if you kill enough), Sven another (melee with a fantastic aoe ranged stun)
Whatever you do, do not play the following heroes when you're new. You will not enjoy the game -
invoker (Skill cap is extremely high, you'll just feed trying to change his 9 spells)
Meepo (Far too easy to die with, lowest win-rate of any hero)
Chen (Extremely team orientated, high micro required)
Wisp (you rely on your team mates for everything, they won't understand what you do)
Possibly Naga Siren (your ult is Extremely powerful but unless your team understands you, you might just save the enemy instead of killing them).
Heroes that absolutely ravage in starter skill games if you're looking to prey on the weak:
Riki (you may end up being called a "Riki-picker") - stealth from 6, no one buys dust early on.
Clinkz (Good damage, easy last hitting with passive, escape, can 1v1 anything after an orchid)
Brood/Dark seer (dominates lanes beyond belief if you dont counter)
Lone druid (if you can micro your bear)