I would like clarification on the "where the seeds come from"
Are plants considered grown after they are harvestable?
You can buy seeds by clicking on the planter, or you can harvest plants that are growing and use one of the fruits or flowers as a seed. Yes, once a plant is harvestable, it's been grown.
Also, the museums I've never built anything on a separate lot other than the one I live on. May the museum be on your lot. Does your household pay for a community lot.
You don't have to buy the community lot. As for building the museum, if you look in your "my gallery" section of the gallery, there's a museum there already. Just put that down and adjust the interior to have the space you need if you don't want to do a lot of building.
Should we keep track of which book were read, which 7 plants, etc. ?
Yes, but more for your own benefit so that you'll know when you've completed the requirement.
The first sim starts as a teen, how do we know when they have positive trait?
You'll get a notification, but you should also check on them. If you pull up the Simonology screen, where their traits are, keep scrolling down and you'll find the character traits section. You'll want to have 3 positive traits, and your Sim can easily slip out of range. I think the Guide has a section on character traits, so look at that for tips on how to develop them.
Good luck!