How do start jungling effectively?

The biggest reason to jungle rather than lane is to increase the total experience available to your team

Your team will need a solid solo laner who can survive a 2/3 vs 1 lane. If you intend to jungle let your team know ahead of time and hopefully someone will pick a good solo laner.

To qualify as a good jungler, a hero needs to be able to maintain an XP/min somewhere between a solo lane/dual lane

Any slower and you'll be critically under leveled. That said, the typical jungling lifestealing hereos, Lifestealer and Wraith King are pretty poor level 1 junglers. Even if nothing goes wrong you'll be behind the curve XP wise and very low HP for a while just because they have no AOE damage, and the lifestealing doesn't keep up with creep damage until about level 5. You may be able to get away with it at low level, but it's a bad habit to get into, as smart players will know you're jungling, and probably low HP, and coordinate easy ganks on you. The really excellent jungle heroes, such as Enigma, Chen, Enchantress, Lycanthrope, Nature's Prophet don't have this problem, they don't have to tank the creeps entirely themselves, and do more damage than just their auto attacks.

If you're dead set on jungling Lifestealer or Wraith King, you'll want to get a Poor Man's Shield, Quelling Blade and either tangos or a salve. Check the rune at 0 minutes, and if your mid doesn't want it grab it for yourself, Double damage or Illusion are extremely helpful at getting to level 3 without constantly being low HP. If it's regen, go back and grab it if it's still there when you're low HP. Start by waiting around and doing nothing until 00:53, and then pull the easy camp to stack it. Just run up and aggro one of the creeps and then immediately run away, they'll follow you and the 1:00 easy camp creeps will spawn. This is very important, as any 2 easy camps will get you to level 2. At 1:13~1:15 pull the double stacked camp into your lane, and your creeps should aggro the neutrals. Let them full engage and then start last hitting the neutrals and denying your own creeps. Send yourself a ring of protection once you can afford it, the armor helps tremendously. Attack speed also helps since you're relying on lifesteal to not die, so send yourself gloves of haste first if you're building Power Treads/Hand of Midas

After you finish the easy camp, scout out the nearest medium camps, the best spawns are the 4 blue dudes or mud golums, since they don't do much damage. If you get unlucky you may just have to wait for the easy camp to spawn again, another downside to jungling with these 2 heroes is they really can't do the difficult mediums or hard camps until level 4~5. Once you hit level 5 you can basically just man mode any camp. With Lifestealer you can triple stack a hard or medium camp and then when you go to do it, wait until you're very low and ult the biggest creep in the camp, will give you a huge boost. Other than that, pay attention to the short lane and coordinate ganks with the dudes laning, check the rune spot, and put wards up if your team hasn't so you don't get ganked.