You don't need a season ticket for the 'official' pub music sessions. They are in pubs so couldn't stop anyone wandering in. Someone might come round with a collecting bucket (and a lot of season ticket holders do come). In the programme they will be marked with price code A (donations encouraged). All events are free if you have a season ticket, most also have a price code for casual attenders A-N, last year N was £14, workshops were £5. Cheaper to buy a season if you want to go to a lot, or pop in and out of different events, but not if you want sessions mostly.
Yes, buy a programme as soon as they come out and for the official sessions do a bit of research in the programme on the session leaders. They all have different ways of running sessions (round the room / jump in / leaders start all the tunes and everyone joins in). And different bias in tunes. E.g. if led by someone who plays Irish music expect a lot of Irish tunes (and other tunes because not everyone who comes will know that), if from Sussex you might get Scan Tester tunes.
May Cheadle runs a "Not quite good enough for a session" where tunes are played at a crawl for people new to sessions or struggling with a new instrument. (I went back last year with new accordian, very helpful)
George Garside runs some excellent sessions for those starting sessioning. He invites suggestions for tunes and if enough people know the tune we play it (led by the suggester, if they want, or someone else). He doesn't let tunes get too fast, just a nice steady pace.
Other sessions may go faster or play less well known tunes.
It would be nice to know who else is running sessions this year and what their specialities are, there is always so much at Whitby that you only find out after you've missed it (or missed something else to go to something which isn't what you expected). Perhaps this thread could be used as a Mudcat joint research project.