A Scout usually joins the Troop for 4 - 6 weeks before being Invested to make sure they enjoy the group and the activities. When they decide to join, they officially become a Scout at a short Investiture ceremony which family are welcome to attend.
We ask Scouts to memorise the Promise which they'll recite (but we're always there for those with stage fright!). Here's a link to download a copy. They do not need to be able to recite the Law however it forms the basis of just about every aspect of Scouting so they do need to be aware of it.