How to set 30% discount on all products in magento shop

Go to Backend , you can see Promotions > Catalog Price Rules > Add New Rule Than enter the information.

Under Rule information ,

status - Active,
Website : Main website
select customer Groups

than under Actions,

you can find more information here :

http://www.hostknox.com/tutorials/magento/catalog-price-rules

http://merch.docs.magento.com/ce/user_guide/marketing/price-rules-catalog.html

http://www.divisionlab.com/solvingmagento/magento-catalog-price-rules/

• Connect to the backend
• Go to Promotions > Catalog Price Rules
• Click Add New Rule
• Rule Name: 30% off
• Status: Active
• Websites : Choose the websites you want to apply the discount for
• Customer Groups: Choose the customer groups you want to apply the discount for
• In the Actions tab, under Discount Amount type 30
• select apply to the "by percentage of product original price"
• Click Save and Apply

Try this one https://drive.google.com/open?id=0BweKwHQ_asV2VE5HTzlLX2RvZUU

here you will get some more examples also which will help you to set some other promotion rule also.