If you need to ask...
The ones you can share your unique experiences and knowledge on. It takes a while to find your feet on meta sites and you learn a lot by watching and reading well received answers. I kinda took a few years to find my feet metawise.
It's better not to answer if you aren't actually sure. It's fine! You'll work things out eventually.
I find, typically comments are best used for clarification. Sometimes they're misused for other things (and I do that) - but stuff like "you might find your post better received if you did foo" works too.
As for answers, it's worth starting with the types of questions you see here.
Personally a good chunk of my own questions are bug reports/feature requests. I call them soapbox questions. An ideal answer here is from the folks who run the sites, but many answers build on my question and add on suggestions. Soapbox questions are discussions and often have a goal of raising awareness about a problem. These could be soft, peripheral subjects (like my complaints about the quality of blog posts) or talking about core site features - like the recent spam epidemic on SU. If it's a bug or feature request - and you have some additional information or suggestions, you can either mention it in a comment (in brief) or as an answer in detail. Monica and Shadow Wizard have good examples here.
On the other hand, a good many questions are about site features and how to use them. A good many of these are under the support tag. You'll find you know these as you learn the site.
As a moderator - I answer many questions on best practices in moderation and how moderation is handled. In general, if it's an angry sounding user, or one trying to dispute a mod decision - stay away.
I've actually found per site meta is a better place to start. I've only really been meta active this past year (of 8 years on the network). In either case, getting the community is useful in successful meta answers.
I wouldn't stress over what I can't answer. I'd look at gaining knowledge from the wonderful folk who hang out around here, learn stuff and at some point, the answers will flow. I didn't have all that much to contribute until recently either.