Add signature:
Open Mail app > left bottom, click the gear icon ( Settings ) > right side, click Signature > select the account you want to add signature or click Apply to all accounts > make sure Use an email signature is turned on > enter your signature into the box.

Automatic replies :
Open Mail app > left bottom, click the gear icon ( Settings ) > right side, click Automatic Replies > select the account > turn Send Automatic Replies to ON.

There are many other options under Settings. Explore them.