You can manually undelete records by click Recycle Bin icon in bottom left menu. Undelete account will include contact, case, opportunity tagged to that Account. But, if you have many records or you have specific record Id or you want to undelete records with some conditions, manually delete is not an option, use Apex code for it.
You not really need to be good developer to utilize Apex code. Here we go:
1. Open Developer Console
From your name in top right, look for Developer Console
2.From Developer Console
Go to Debug menu and select Open Execute Anonymous Window
3. Enter SOQL
Account a = [SELECT Id FROM Account WHERE name = 'singtel' ALL ROWS];
Important to add ALL ROWS, otherwise deleted records will be not return on query.
4. Monitor Apex Debug Log
Go to Monitor - Logs - Debug Logs (in Salesforce menu, not Developer console)
Add user to monitor, default it will capture 20 log request for the users
Click Execute button in Enter Apex Code window
6. Back to Debug Log
Select latest debug log and click View to monitor