So... at the beginning, you should have 41ap, use it to harass other heroes and get the first blood. Try to lane with a tank. Usually, before 16minutes you should have a Rod of Ages if you don't buy the boots, if you do get the boots, you'll have the Rod around 20minutes.

Always cast Miasma and Noxious Blast at the same time (use shortcut keys), enemies will be slowed and poisoned. Then run after them, don't be afraid. Cast Noxious Blast in front of where they are going because of the small casting delay, it should hit the target almost every time.

Use Petrifying Gaze for defense when melee heroes attacks you (miasma, petrify, blast, fang, blast, fang...)