As a regular participant since 2008, I heartily recommend People's Remix Compo as an easy-going environment to hone your skills, no matter what your current level is. We could always use more participants!
I remember a regular voter who was ultimately disappointed in how relaxed the competition was and felt the musicians weren't taking it seriously enough because they weren't fighting tooth and claw for the win. I also find the feedback is on the kind side, though still useful.
I trust MnP is much the same, even more so. Since its idea is to stick close to the source, there's definitely no composition skill prerequisite. And even the tournament compos can definitely be entered at whatever skill level. Also look for Remixing with the Stars, which is about pairing up with a mentor.
Personally if I wasn't taking part in PRC (and lately other compos), I wouldn't probably have ever even started a VGM arrangement let alone finished several. Regularly having a deadline and some sort of an assignment has been really really good for my productivity, and through that has come the gradual skill increase.
I definitely ditto the recommendation for Rozovian's guide, it was very helpful for me as well!