Space travel is a tricky thing. Even with the most hospitable traveling environment and a near-perfect journey, space is just not great for humans to be in it. 100 years is a long time for a small society to function. In order to give that its best chances, an excellent crew and passengers will be an absolute necessity.
I offer that selection should include the following, and in this order:
1. Psychiatric stability
Space is unbelievably huge and empty. Also, it isn't Earth. There is a level of terrestrial attachment that almost every human has. Leaving that behind to enter the empty dark is a scary thing. Thus, the crew and passengers must possess a stable psychiatric profile. This should even be reviewed one or two generations back, as predispositions for mental instability can be seen in family. At the end of the day, no matter how good a candidate is, if he can't handle the concept and nature of space travel, he'll be more of a danger than an asset.
2. Physical Stability
Space Travel is physically difficult. Getting into orbit alone requires exposure to at least a couple G's of force for a short period, potentially more. Once in space, dealing with the potential affects of low-g or null-g exposure require constant training and exercise. If the candidate's physique can't handle that for his or her lifetime, it's a no-go.
3. Appropriate space-faring Skills/knowledge
Being successful in space means having at least a couple of people that can keep everything up and running. Everything from engineering to navigations needs more than a couple people that can make everything function. This may mean more front-end training, but it is very important that there be a solid operational crew with enough bodies to put two people on each of three 8-hour shifts at minimum for every major system and operational component. This is a last hope scenario, backups and constant readiness are a must.
4. Other Appropriate skills/knowledge
If you're going to need someone to do a job once they land, you'll need at least one or two people for each of those jobs on board while they travel. Naturally, these people will have to train a couple children en route to the planet, because it is likely the first generation will die off, thus it will be important these people have access to training materials and the like. However, the sole emphasis should not be on teachers. There should a good smattering of working professionals for every job to ensure that experience speaks into the raw knowledge aspect as well. Bonus points for those jobs that can train while also fixing actual issues during travel.
5. Diversified Background/age/gender
The gene pool will need to be pretty diversified for effective procreation both en route and on arrival. It will be beneficial to have several walks of life on the ship to encourage multiple problem-solving approaches. And there should be some families, some singles, some married with kids, etc. to get the full effect of all that stuff that only these different groups can add to the experience of the entire crew. In a perfect world, you'd also wants these people to be able to spend a little time together before leaving, so quirky differences can be worked out early, or used to disqualify candidates that would behave negatively with others.
6. Strong predisposition for disseminating information
Admittedly, this last aspect is a bit of a fail-safe and sort of difficult to quantify. The candidates should be able to easily and rapidly acquire and make use of new information. This is sometimes thought of as intelligence, or IQ, or even common sense; it depends on your outlook. The crew - being relatively small when compared to the whole of the human population - will very likely need or want to learn some things that you can't pack a professional onto the vessel. For these things that will either go forgotten or considered inconsequential, it would be nice to have a video library that the crew will have access to for self-study or as supplementary to normal education during the journey and on the new planet.
I feel this is a difficult thing to answer without biasing the creative process, as the basic elements of structure will either dictate or will be dictated by narrative elements. With that in mind, some things to consider:
- During travel, renewable food and water resources will be essential
- Upon arrival, resources for finding/hunting/harvesting food will be essential, along with knowledge on their use
- The society will need some way for the various people on board to make informed decisions either through representation or direct democracy. Consider a system that utilizes some sort of elected triumvirate paired with a board or other power checking organization.
I hope this is helpful in jump-starting some thought.