agent said if you can put off A' offer...

but I am not working now, and the market...I thought about postphone the start day, but they know I am not working now and cannot find a good reason.

I thought about looking for other opportunities even after I started at company A. so the question is,
How to make other company think you are 'loyal' that if you started in a company in short time but leave for a better offer?
When you leave a company in short time, do you have to give two weeks notice and what should you say as the reason for leaving?