Please review the REST API 2.0 guide on here.
You can also find numerous samples with the REST API in other posts in this forum.
Note that the session ID is to be sent in the authorization header of each message.
If you need further assistance, don't hesitate to post or comment again. Please specify what platform/language you use and what you want to achieve with the API so we can come up with answers that are most relevant to your scenario.
Hope this helps,