Candice,
I just did a quick search on Air Asia and you do need to make a decision. I tried both Ranong & Krabi. Some dates were already full, others available but pricey. Last minute travel is more expensive than giving them 2 weeks notice.
I have been to Krabi, not Ranong. Krabi has heaps of hotels & travel shops right in town. I am sure they could oragnize a tour to Khao Sak N.P once you are there. If you choose Krabi, take a taxi/truck into town and start walking the streets. Plenty of GHs and hotels to choose from. Not very big, you can walk the main streets in 15 minutes. If you do book a flight, then you can book a room for the first night if that makes you more comfortable. I always arrive and wing it, but others do not like that option.
You do have another choice if you don't book a flight. There are overnight buses leaving from the Southern Bus Station in BKK for Surat Thani in the early evening. You arrive in Surat very early am. Figure 500-700 baht for this leg. Buses leave for Khao Sak every hour from 6:00am on. 1 hour, 140 baht to Khao Sak. The are also overnight buses at the same station and around the same time for Krabi. Figure 600-800 baht for the bus to Krabi. Takes 10-12 hours.
This site seems to have plenty of good info. Good luck to you.