Before I go into detail, I would like to point out that I have never GM'd before and I've only roleplayed in 1 campaign of Pathfinder. My roleplaying knowledge is very limited and even more so with this.

Ok, so I'm looking at the Talent Tree for Assassin. The first question is: Do I add ALL of the Career skills and ALL of the Bonus Career skills or do I choose some of them? If so, how many?

Secondly: I noticed that the talents have a cost. What are the costs? As in, what am I spending to give the character these skills?

(I'll be asking a similar question about how tables 2-1 through to 2-3 work for Obligation, so inbox me on that before I get a chance to post it)

The player picks 4 career skills from the main career, and 2 bonus career skills from each specialization. So 4 for Bounty Hunter and 2 from the Assassin list. Droid characters get a 6/3 split.

The talents are bought with xp, either the base amount that is provided at character creation or via what is rewarded each game session by you to the players. The fairly well agreed upon approach though is to spend the starting creation xp mostly on attributes and maybe a bit on skills, talents come fast and cheap enough even after one session.

I'm not being surly but if you read the character creation section some of this will be clearer, not all, because lord knows we butt heads here over rulle intent/interpretation.

The player picks 4 career skills from the main career, and 2 bonus career skills from each specialization. So 4 for Bounty Hunter and 2 from the Assassin list. Droid characters get a 6/3 split.

Not to be contrary (especially considering your note about butting heads) but my take has been that all of the skills under Career and Specialization count as Career Skills, but you only take free ranks in four of the ones listed under Career and then two of the ones listed under Specialization.

As for talents, the costs are listed at each level of the Talent Tree. The uppermost level is 5 XP, and each subsequent level increments by an additional 5 XP cost. 2P51 is correct that traditional wisdom is spend your starting XP on attributes and core skills, then use XP from later sessions to pick up talents.

Captain Raspberry, you are correct. The eight skills listed for your Career and the four skills listed for your Specialization are all considered "Career" skills for the purposes of calculating their cost.

Then you may acquire free ranks in four of the Career skills and two of the Specialization skills, with Droid characters gaining free ranks in 6 of the 8 Career skills and 3 of the 4 Specialization skills, respectively.