WHere to leave the motorbike...my plan is to take a bus from klang and sukhumvit place to BKK. Can I just leave motorbike at bus station or will somebody steal it?

There's a possibility that someone will steal it, there's also an equal possibility that someone will not steal it.  How could anyone here know what is going to happen unless they're planning on stealing it?

If leaving from North road station,

leave it with secured parking;

not expressive.

Nobody will steal it.

This is not your home country.

where is secured parking i just wanted to leave it in a parking place where condo is next tu bus station

You won't find space on the bus station itself. Is there still 'secure parking' right next to the bus station? Mind you that is from Rung Ruen on North Rd, I would always choose them as it is a coach not a dangerous minibus. Not familiar with a bus station on Klang.

Many times I popped into Bangkok and came back the same day from there. 

i leave my bike at the parking next to and just before North Pattaya Bus station, 30 baht a day, looks secure to me. Better to get these buses too, big ones are safer, they go to Ekamai or Mochit

In case that I don't take Bell Travel to the airport and travel to the city by the Airport Link, I always leave the motorbike at the parking next to the North Pattaya Bus Terminal. No problem since years.