How do know which lane to pick?

Lane usually depends on the hero you are playing, and also the "role" you'll take on.

First of all, the safelane and offlane, or the easy lane and the hard lane are nicknames for lanes. The safelane for your team is the longer lane that has the majority of your jungle running alongside it (bottom for radiant, top for dire). Traditionally the safelane or easy lane is farmed by your team's "hardest carry", whomever needs the most farm. This is because it is easier to secure last hits in the safelane. That hero will often be joined by a support in lane.



The other lane, the hardlane or offlane, is generally considered more dangerous for your team, because it is easy for your enemy to hide in their own woods and surprise you. In pro matches you often see one person solo the offlane in order to get solo experience. This person is usually playing a hero that "needs levels" rather than farm (a hero like clockwork who is capable of killing people once he has his hookshot).

In most pubs, you will mostly see 2 dual lanes in which case going top or bottom, or safelane or offlane, doesn't make a huge difference.

In terms of jungling, it's rare that a hero can start in the jungle and make the most out of it. Sure a hero like Lifestealer or Bloodseeker is capable of surviving in the jungle starting at level 1, but generally it is a bad idea because it makes the other lanes weaker. Chen, Enchantress, and Enigma are almost the only heroes who can start in the jungle and get meaningful gold and experience. In fact oftentimes this heroes will rotate out of the jungle early game to help get a kill or push a tower.

Best to go offlane when:

• You have an easy escape and do not rely heavily on farm: Mirana, Batrider, Riki, Bounty Hunter, Broodmother, Clinkz, Timbersaw, Weaver, Windranger, Slark or Dark Seer. (Some cases: Faceless Void, Queen of Pain, Nature's Prophet, Storm Spirit, Puck, Nyx Assassin).
• You are tanky or you can really mess up their safelane: Abaddon, Axe, Earthshaker, Sand King, Spirit Breaker, Bristleback, Tidehunter, Treant Protector, Centaur Warrunner, Chaos Knight, Clockwerk, Undying
• Good solo offlanes: Bristleback, Clockwerk, Centaur Warrunner, Timbersaw, Weaver, Broodmother, Clinkz, Lich & Dark Seer.

Better to go safelane when:

• You are a core hero which relies (heavily) on farm: Anti-Mage, Drow Ranger, Faceless Void, Juggernaut, Luna, Medusa, Morphling, Naga Siren, Phantom Assassin, Phantom Lancer, Sniper, Spectre, Sven, Terrorblade, Troll Warlord, Ursa, Wraith King
• You are a (slow) support with no escape
• You are a support with stuns (First blood potential)
• You are a support which is good at babysitting a carry: Abaddon, Dazzle, Io, Omniknight

You go mid when:

• You can easily gank other lanes with level advantage of solo lane xp: Storm Spirit, Invoker, Puck, Templar Assassin, Skywrath Mage, Pudge
• You/your team profits heavily if you have a level advantage: Lifestealer, Gyrocopter, Bloodseeker, Huskar, Outworld Devourer

You pick a jungler if:

• The enemy team has a weak laning phase (lack of stuns, they have lots of melee and you lots of ranged).
• Your solo offlaner will benefit from level advantage (Bristleback, Weaver, Centaur Warrunner, Nyx Assassin)
• When you see enemy picks always think of where you think they will go. They pick a melee safelaner? Might be a good idea to pick a ranged offlaner (Mirana, Clinkz, Windranger). They pick a ranged offlaner with a ranged support? Might be better to something tanky like Bristleback, Timbersaw or Centaur Warrunner (Return damage).

Might have missed some hero's but this should give you an idea.