Start by reading the avast help file Start > All Programs > avast! Anti-virus > Help. Make your self familiar with the program to get the best out of it.
A search for 'boot scan' will return many hits, as this has been discussed many times. There is a wealth of information on these forums available to all.
Right click on the avast icon and select start avast! anti-virus, her you can start regular scans and boot scans (OS dependent), what OS are you using? Pressing F1 should also bring up the help file or click the Menu or right click and you have access to settings, schedule boot scan, etc. etc.