Im doing this in my spare time so havent really looked into it too much....

Ive been asked to supply a SSL certificate for a website Ive had a hand in setting up, but I know fek all about SSL certificates really, so need some advice on what I should be getting/doing...

Currently the main website is hosted on OpenHost and has a LetsEncrypt SSL installed via Plesk.   (www.mydomain.com for example)

However the web application on the same domain name, is on a AWS instance (myapp.mydomain.com).

Will a wildcard cert cover everything ? Or is it based per host ? What should I be looking at ? Who should I buy from ? 

Thanks :)