Firstly, I'm not aware of how RSI is related to sitting, or how standing would alleviate RSI. RSI is due to many things: posture, positioning of mouse/keyboard, length of work/breaks, and of course, how repetitive your movements are. Given all that, standing or sitting is just a small part of RSI, and possibly the least significant factor.
If your posture is right, if your desk, mouse and keyboard are at the right height and distance, if you take regular breaks, and if you exercise your hands/fingers, then it makes little difference whether you're sitting or standing.
That said, I've recently set myself up with a standing desk – just an Ikea Fredrik. I just want to try it out for myself too, to see whether I like it. I do kung fu, and my main motivation was to practise my stance for longer periods. It's a stance similar to if you were sitting on a bar stool – that is, a few inches lower than standing straight-legged, with hip tilted straight.
That stance has taken some years for me to be able to do for more than 20-30 minutes without turning into a jelly-legged mess. If you're standing straight-legged, this won't be a consideration for you. So, I'm in reasonable shape, but by no means super fit.
This bit you add – "I'm not very used to standing and walking, and I'm afraid of being tired all the time and having back and feet pain..." – is a really important point. Don't expect to be able to just start working at your standing desk for extended periods. Go and do more walking at the very least to build up a bare minimum level of leg/back strength and stamina.
A standing desk is a great motivation to sort out your level of fitness. How long should you try the standing desk before giving up? That depends. Give yourself time to get used to it. Don't stand before you can walk (to paraphrase "don't run before you can walk").
Good luck with it, and please do report back after you've tried your standing desk for about a month. If you keep a blog, a blog post about your experience would be great.