If you can answer the question and have a fair bit of confidence in your answer, then that's all it takes would be my answer. Thus, it is a matter of having both technical know-how and confidence to make the post.
Just don't forget that a lot of people with really high reputations on those sites took some time to build that reputation and so it may take a year or two to get the reputation up by answering questions well enough for them to get up voted. Just remember to be respectful of people posting questions and be careful on giving smart alec answers as those can sometimes get down voted or deleted I'd imagine if they are sufficiently hostile.
Lastly, do be aware of how well you can type sentences as this can be another matter as sometimes it can be hard to decipher what was posted if the sentences seem to follow a haphazard logic.