For Sandbox Mode - To get Api keys and other data related to paypal you have to sing up for it.First go to this link – https://www.paypal.com/webapps/merchantboarding/webflow/unifiedflow?execution=e1s1 and this page will open then sing up here.
after sing up ,click on verify link in your inbox then go to https://developer.paypal.com/webapps/developer/applications/accounts and log in here and you will see your account like this
you can add and delete accounts here, now click your account and you will see a Profile link then click on that
after this you can see your api related data in API credentials.
And please wait atleast 12 hours after verification your mail, you can get merchant id and keys after login paypal account.
For Payment Mode - This process is little complicated so you need to follow the guidelines as per the below screenshots, after login here https://www.paypal.com/ please go to this link or if you don’t have account please create one and after verify your mail and complete tasks
then go to this url -https://apps.paypal.com/user/my-account/applications and select options like this
