Can I secure subdomains also with a single SSL Certificate?

Yes, you can secure all your sub domains along with your primary domain by purchasing the Wildcard type SSL certificates. To secure multiple sub-domains with single certificate you have to purchase any one of the following certificates:

1) Comodo’s PositiveSSL Wildcard

2) Comodo’s EssentialSSL Wildcard

3) Comodo’s PremiumSSL Wildcard

4) Geotrust’s RapidSSL Wildcard

5) Geotrust’s True BusinessID Wildcard

The above certificates helps you in securing all your sub domains with a single certificate.