Welcome to Super User!
First of all, let's distinguish between two things: Upvoting an answer, and Accepting an answer.
Since the question you linked to was not asked by you, you cannot (with any amount of reputation) accept the answer. You can, however, upvote it if you have enough reputation.
Upvoting means clicking the little "up" arrow to increase the number of votes for the answer. Accepting the answer means checking the checkmark next to an answer, but only the person who asked the question is allowed to do that (and nobody else can override the decision they make).
The privileges on Super User at different reputation levels are described here. The privileges are put in place for a number of reasons:
- To eliminate cheating, such as someone creating a lot of user accounts and using these new accounts to upvote their own questions/answers.
- To prevent spammers and people who ask bad questions (which we don't want on the site) from artificially inflating the perceived vote score of questions and answers they post.
- To encourage users to participate in the site, privileges are unlocked as your reputation increases as a way of "gamifying" the participation process.
Here are some ridiculously easy ways to get reputation at the low numbers (15 and 50) you're talking about:
- Each upvote on a question you ask gives you 5 reputation.
- Each upvote on an answer post you provide gives you 10 reputation.
- Each accepted suggested edit gives you 2 reputation.
So, to get to 15 reputation, you'd have to get three question upvotes, two answer upvotes, or 7 accepted suggested edits, or any combination of these things. Many users can get past 15 reputation (regardless of their knowledge level) in about 20 minutes.
To get to 50 reputation to be able to make comments, you'd have to get 5 answer upvotes, 10 question upvotes, 25 accepted suggested edits, or any combination of these things. Many users can get past 50 reputation (regardless of their knowledge level) in about an hour.
One thing we want to emphasize is that if your only purpose in participating in our site is:
- To make comments like "Thanks, that helped!" or "This worked for me!";
- To upvote answers that worked for you
... Then you won't earn any reputation from either of these actions, so you'll never get enough reputation to perform either of them. It's kind of a "chicken and the egg" problem in that sense: the things you're wanting to do, don't provide reputation when you do them, either.
Well, simple. Comments like "Thanks!" are actively discouraged, because they just clutter up the site, and are not needed. If an answer is useful to you, that's great! Just leave it at that.
You don't have to edit a Wikipedia article with a comment saying that "I learned something today!" every time you read an interesting Wiki article, do you?
Do you call up the local news station every time they provide an intriguing news story to tell them that it was a good one?
Upvotes and comments are reserved for users who are taking a more active role in the site, primarily by: (1) posting answers to questions; (2) asking good questions; or (3) editing posts that need revision. Once you spend even 1 hour working on any of these three things, you'll easily have enough reputation to upvote and make comments.
Then, once you can upvote, feel free to do so whenever you find a question or answer that you find useful. But please don't add comments indicating that the answer helped you; they just clutter up the site.
Comments are for asking for clarification, providing constructive critique, or making the community aware of inaccuracies in an answer. So it would be okay to post a comment if an answer doesn't work for you, if you explain why and what circumstances it doesn't work under.
With 8 reputation, you're more than half-way towards having 15 reputation, and once you get that far, getting to 50 isn't difficult at all. If you intend to use the site even occasionally, it might be worth putting in the effort to get up to a modest reputation level. But if you don't, it's really no harm done.
About 90% of all Super User and Stack Overflow users don't even have an account, so they:
- Can't post questions, answers, or comments.
- Can't upvote anything.
- Have never participated in Meta (you've already done this!).
And that's perfectly fine with us. We're happy to collect, prune, and take care of a body of knowledge that the vast majority of the people benefit from, but don't have to contribute to. If you feel you want to contribute, we welcome you; but just keep in mind that by even having an account, you're in the top 10% of our users!
The low-reputation privilege restrictions are mainly there to make sure you're a human being (and not a bot / computer script), and that the quality of your communication is up to the standards of our site. Please pardon our lack of trust at this early stage; the veil will quickly be lifted once you post a few questions, answers, or edits.