How to put together a showreel if you haven't done much film work
The best way to put together a showreel is to find some interesting scripts from either films, TV or theatre which you believe will showcase your best talents.
Use a few different scripts which will also show your diversity as an actor. Therefore, you might want to use a monologue from a film like 'Love actually' which might show humour, or a duologue from a TV show like Corronation Street or Shameless. The more different each script is, the stronger your showreel becomes due to there being varied emotions throughout. You want to show who ever is watching it that you have many strings to your bow and have the capability to produce different emotions well on camera.
Showreels don't need to have professional, paid work on them. As long as your showreel shows off your acting skills, it doesn't matter how or where you performed it. A lot of actors do this nowadays as it is so hard to get a professional, paid role in the industry with there being so much competition. Taking scripts from films, TV and theatre has become more and more common and also still looked upon as professional in the eyes on a casting director / agent. It is still doing the same job at the end of the day and hopefully this will land you your first big role!
In terms of filming this, the best way is to find someone who has an clear camera (photographer or someone in the filming industry) to help you out and film you. This will also be good for their showreel as much as yours! And the easiest way to put a showreel together for free is through Movie Maker. If you don't mind spending a bit of money to get it put together professionally then do that for the best results, but I know a lot of people who do it themselves and it always looks good in the end.