How does the billing work I am unable to subscribe

The following document lists Lightsail's rates.
Note that Lightsail prices vary depending on specifications and operating system.
For example, in the case of Linux, the cheapest fee is 3.50 USD per month.
Also, if you delete Lightsail in the middle of the month, you will be billed by the hour.
https://aws.amazon.com/lightsail/pricing/?nc1=h_ls

I would suggest subscribing to amazon free tier first here: https://aws.amazon.com/free/?all-free-tier.sort-by=item.additionalFields.SortRank&all-free-tier.sort-order=asc&awsf.Free%20Tier%20Types=*all&awsf.Free%20Tier%20Categories=*all

Billing is determined on how you utilize AWS resourses. You can use skillbuilder to get to know AWS fundamentals and build your knowledge from there. You can utilize the free resources on amazon skillbuilder. Try this for a start :https://aws.amazon.com/free/?all-free-tier.sort-by=item.additionalFields.SortRank&all-free-tier.sort-order=asc&awsf.Free%20Tier%20Types=*all&awsf.Free%20Tier%20Categories=*all