Hiyas! Hmm, it's kind of hard to explain. "Öva" is (to) practise and "öva mig/sig" means (to) practise myself/yourself, but they translate to about the same thing. Both are used, but in different situations. "Jag övar på svenska" (I'm practising Swedish) is the correct way of saying it. "Jag övar mig på att kunna förstå svenska" (I'm practising trying to understand Swedish) "Jag lär mig Svenska" (I'm learning Swedish). Saying anything else is wrong. I guess it mostly depends on what verb is used. Swedish can be quite difficult...

'Öva sig' is to practice details, ment towards perfection. 'Öva' is a bigger picture or could be less serious.