Installing Certbot:

$sudo apt-get install certbot

Registering the domain and subdomains

$certbot certonly -d 'ntalam.com' -d '*.ntalam.com' --manual --preferred-challenges dns

Follow the instructions and paste the random text in your TXT registers. 

Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator manual, Installer None
Enter email address (used for urgent renewal and security notices) (Enter 'c' to
cancel): me@ntalam.com
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Please read the Terms of Service at
https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf. You must
agree in order to register with the ACME server at
https://acme-v02.api.letsencrypt.org/directory
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(A)gree/(C)ancel: a
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Would you be willing to share your email address with the Electronic Frontier
Foundation, a founding partner of the Let's Encrypt project and the non-profit
organization that develops Certbot? We'd like to send you email about our work
encrypting the web, EFF news, campaigns, and ways to support digital freedom.
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(Y)es/(N)o: y
Obtaining a new certificate
Performing the following challenges:
dns-01 challenge for ntalam.com
dns-01 challenge for ntalam.com
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NOTE: The IP of this machine will be publicly logged as having requested this
certificate. If you're running certbot in manual mode on a machine that is not
your server, please ensure you're okay with that.
Are you OK with your IP being logged?
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(Y)es/(N)o: y
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Please deploy a DNS TXT record under the name
_acme-challenge.ntalam.com with the following value:
i7fH5wyCtytoo************************
Before continuing, verify the record is deployed.
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Press Enter to Continue
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Second text…

Please deploy a DNS TXT record under the name
_acme-challenge.ntalam.com with the following value:
Rx1z4cxnb81VpZ************************
Before continuing, verify the record is deployed.
(This must be set up in addition to the previous challenges; do not remove,
replace, or undo the previous challenge tasks yet. Note that you might be
asked to create multiple distinct TXT records with the same name. This is
permitted by DNS standards.)
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Press Enter to Continue
Waiting for verification...

Certification validated

Cleaning up challenges
IMPORTANT NOTES:
- Congratulations! Your certificate and chain have been saved at:
/etc/letsencrypt/live/ntalam.com/fullchain.pem
Your key file has been saved at:
/etc/letsencrypt/live/ntalam.com/privkey.pem
Your cert will expire on 20*******. To obtain a new or tweaked
version of this certificate in the future, simply run certbot
again. To non-interactively renew *all* of your certificates, run
"certbot renew"
- If you like Certbot, please consider supporting our work by:
Donating to ISRG / Let's Encrypt:   https://letsencrypt.org/donate
Donating to EFF:                    https://eff.org/donate-le