Tickets to islands, koo chang etc

Hi,


Been in bangkok for about two weeks now, will be moving to huahin soon but before that I wanna do some beach experience. 

I have been looking at going to koo chang with bus and ferry. 

Is 12goasia, a good website to order or should I find like a travel group somewhere in bangkok? 

Otherwise do you have any other tips or somewhere I could go?

Pattya seems easiest but want a bit more beachy, island vibes I guess. Just for a few days. 

Edit: And sorry, if you know any smooth and easy luggage storage. Im center of bangkok, near all the temples etc. 

Thank you and any information is appreciated, thanks :)