Accountcontroller.apxc

public class Accountcontroller {
	
    @AuraEnabled
    public static List<Account> getAccounts() {
        return [Select Id, Name, Industry, Phone from Account limit 10];
    }
    
}

lightningDataTableSample.cmp

<aura:component controller="Accountcontroller" implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,forceCommunity:availableForAllPageTypes,force:lightningQuickAction" access="global" >
    <aura:attribute name="mydata" type="List"/>
    <aura:attribute name="mycolumns" type="List"/>
    <aura:attribute name="selectedRows" type="List"/>
    
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>
    
    <lightning:datatable aura:id="mytbl" data="{!v.mydata}" 
                         columns="{!v.mycolumns}" 
                         keyField="Id"
                         selectedRows="{!v.selectedRows}" 
                         onrowaction="{!c.handleAction}"
                         />
</aura:component>

lightningDataTableSampleController.js

({
	doInit : function(component, event, helper) {
       
        component.set('v.mycolumns', [
            {label: 'Account Name', fieldName: 'Name', type: 'text'},
            {label: 'Phone', fieldName: 'Phone', type: 'phone'},
            {label: 'Industry', fieldName: 'Industry', type: 'text'}
        ]);
        
		var action = component.get("c.getAccounts");
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (state === "SUCCESS") {
                console.log(response.getReturnValue());
                component.set('v.mydata', response.getReturnValue());
                // trying to set the selected rows 
                var selected_rows = helper.getSomeRows(response.getReturnValue(), 3);
                var selected_rows_mod = ["00161000002K6BbAAK", "00161000002K6BcAAK", "00161000002K6BdAAK"];
                component.set("v.selectedRows",selected_rows_mod);
            }
        });
        
        $A.enqueueAction(action);
	}
})

lightningDataTableSampleHelper.js

({
	getSomeRows : function(objs, cnt) {
		var selected_records = [];
        
        for(var i=0; i<cnt; i++){
            selected_records.push(objs[i].Id);
        }
        
        return selected_records;
	}
})