Create a self signed cert, convert .pem to .cer

Create private key:

openssl genrsa -out privkey.pem 2048
 

See what's inside the private key:
openssl rsa -noout -text -in privkey.pem

Create public key using the private key

openssl req -new -x509 -key privkey.pem -out cacert.pem -days 3650

See what's inside the public key:
openssl x509 -in cacert.pem -text

Convert public key from .pem to .cer

openssl x509 -inform PEM -in cacert.pem -outform DER -out a.cer