Hi Gemma,
You will find some useful information about the genus Serissa at these two sites:
1.
BCI's species index.
2.
bonsaisite's serissa page
Fertilize Serissa every couple of weeks during the growing season with Miracid. Please see the Bonsai Faq on this board for suggestions on watering. Watering on a schedule isn't really practical with most bonsai, Serissa in particular. If you are keeping the tree indoors you'll almost certainly need some extra light in the form of a grow lamp.
The browning leaves are probably not a result of the drive home. If you were comfortable in the car, your Serissa should be fine too. We've experienced some scorching of azalea foliage in unventilated vehicles, and it is unmistakable, affecting just the upper portion of the tree where the air was hottest.
Serissa is very prone to dropping leaves in response to changing conditions. It's important not to overreact to this. Keep maintaining the soil moisture at a level that is just slightly moist but never soggy. Water thoroughly when needed.
Happy Growing!
Dr.Bonsai