Get The Endpoint
You need the instance endpoint if you want to execute direct REST calls.
Go to Settings > Customizations > Developer Resources and copy the Service Root URL and use it as [Organization URI] in the examples provided by the documentation.
Source: Query data using the Web API
Basic Retrieve Multiple
Use the method Xrm.WebApi.retrieveMultipleRecords to retrieve multiple records.
Xrm.WebApi.retrieveMultipleRecords("account", "?$select=name&$top=3").then(
function success(result) {
for (var i = 0; i < result.entities.length; i++) {
console.log(result.entities[i]);
}
// perform additional operations on retrieved records
},
function (error) {
console.log(error.message);
// handle error conditions
}
);