You can go to Koh Jum by boat from Leam Kruad pier, the ticket costs 100thb one way and the boat leaves about every 1,5 hours (see schedule in pics). From Krabi town you can catch a songthaew to Leam Kruad pier, but you need make one change on the way.
If you booked a room already, I suggest calling the resort and ask if they can pick you up from Mu Tu pier on Koh Jum.
We stayed at Jungle Hill bungalows which offered affordable bungalows by the sea, highly recommended. Good kitchen as well.
What to do on Koh Jum? Not much, just relax. Rent a motorbike from the resort and explore the island. Swim a lot, the water is clear, and you have plenty of beaches to choose from.