Hello and welcome.
Here is a link to the forum care sheet which is always a good place to start. It will answer many questions about setting up a home for the snake and its care.
Garters usually require a settling in time so that they feel secure. If it were me I would wait a few weeks before handling. Wait until it has a few weeks of eating under its belt.
Handling can come after the snake is used to its new home.
Just my opinion.
Garter Snake Forum - Garter Caresheet