I hv 150CC scooter. When I start it, and turn the throttle to give it gas - it quits.How to tighten cable?

I need detailed instruction on how to remove the hand grip and tighten the throttle cable - or maybe the hand grip does not need to be removed?

Tightening your cable is not going to solve your problem.
You are experiencing a fuel delivery problem.
Without more details I can only offer you a few possibilities.
1) clogged or restricted fuel line
2) clogged jet(s)
3) if your fuel intake is vacuum actuated you may have a vacuum leak.

better thing you take the bike to a garage!!

You might not want to tighten the cable. It might be quitting because it is getting too much gas, in which case tightening the throttle would make it worse. How does it behave otherwise? high or low rpm's under normal conditions?

If, in fact, the throttle cable needs to be shortened, which it probably doesn't, you most likely are looking at the wrong end of the cable. It should be shortened at the carb. side.

You may be abe to tighten it at the carb. There should be a lock nut where the cable attaches to the carb. Loosen it, take up the slack, and tighten it.

But before you do, check if your idle and fuel mix screws on the carb are properly set.