In addition to what serverco wrote above, there are two things that might come up which it can't hurt to know now
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Let's Encrypt does not currently offer "wildcard" certificates. So you will need to be able to list all the domains you want a certificate for, you can't (as you can with some of the pricier paid certificates) get just one that works for every possible name in your domain. With Let's Encrypt you'd need to issue new certificates for any new names you needed.
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Let's Encrypt is rate limited. This won't be a problem if, say you make one cert, then change your mind and make two more. But if you have plans involving dozens of names or names that change every day, you will need to read the rate limits