What kind of bike is it? Can you post a picture?

If you're not bad at mechanics, it shouldn't be hard to get it to stop "jumping gears" all the time. That's really not a good reason to convert a bike to fixed gear, unless some of the parts are actually broken or worn out.

If you do want to convert it to fixed, the most expensive thing you'll need is a new rear wheel or wheelset to thread on a cog and lockring. Single speed conversions are much cheaper because they can involve nothing more than threading an SS freewheel onto the hub, or a single cog on a Freehub if you have one of those. A wheelset will most likely be at least $100. But I'm not sure if that means anything to you in the Netherlands.