Guten tag! Welcome to the site! As others have said, build summat cheap and get to the RobotsLive / Roaming Robot events. you'll learn loads. I just got back to building feather number 2 after the first one got badly damaged at it's first event haha. It's common practise to build a drill powered rammer. It involves a couple of cheap drills from argos, cheap speedo and a couple of tamiya 7.2V batteries taped together. You can even get a cheap radio set for 25 notes. Build a basic body (plastic, wood, steel, ally all commonly used) and chuck your running gear in. I came back with my electrics intact so they're going into number 2. I think it's like anything, it's as dear as you make it. As with anything, you won't get to the moon and back on a fifty quid robot but it'll get you too some events for a laugh. I'd recommend that, and as you progress more can spend more if you want to. Just keep it simple and have a read round and remember there's no such thing as a stupid question.

Dangerous Dave