Server advice for RoR for newbies

Why not just do it on Heroku or a similar system? Then all the
difficult stuff is done for you.

If you want to do your own and it is to be a public site then unless
you know a lot about the security issues of setting up a server then
you should be very concerend. It is not something to be taken

Further again, if you decide to go ahead, when you say no more than
100 users, it is not the number of users that is significant but how
many are expected to be concurrently making requests on the system.
If you are expecting less than one request per second then you could
likely do it with a Raspberry Pi or similar.