There isn't an easy way to learn which accent you must use.
You have to know them by heart. If you read French it will come easy to you.
There is one easy thing for YOU ENGLISH PEOPLE.
^ ^ ^ accent circumflex : It is often used because an english word became French it is instead of the S
Examples: Forest = forêt, Hospital = hôpital....
Watch those links:
Thanks to DBRS01 , she found those websites.