How do I get my rep up quickly? I've seen a couple of questions I could answer, or answers I could comment on to give, help but I can't
I also don't like how you need 300 rep to create your own tags.
High-quality, useful content.
That doesn't seem terribly helpful, but it's the only strategy that consistently works.
Basically, there are three ways you can earn rep. Note that a lot of this is covered in the help center page What is Reputation? How do I earn (and lose) it?; actually a lot of everything is in the Help Center. Read the Help Center, it's helpful.
Provide high-quality answers. How do I write a good answer? is your guide. You mentioned that you've seen questions you can answer; that's great! Answer them. There's no rep requirement to post an answer1, so if you feel like you have a good answer (or feel like the existing answers have missed something), post it. Just be careful to make sure that what you're posting actually answers the question; we want to keep comments separate from answers, but it doesn't take long before you can comment.
Note that high-quality answers are generally a faster way to gain rep than high-quality questions; an upvote to your question nets you 5 rep, while an upvote to your answer nets you 10. The flip side is that high-quality answers are generally harder; they require much more research.
Edit posts. There's no rep requirement to suggest an edit to another user's question or answer; until you reach 2000 rep, your edits are reviewed by other members of the community. If two members agree that you proposed a good edit, you'll get 2 rep out of it. That may not seem like a lot, but it's a good way to gain rep if you don't feel confident asking or answering.
However, note that you can only gain 1000 rep from proposed edits; if you make 501 good edits, before reaching 2000 rep, you won't earn anything for the last one. It also stops working once you hit 2000 rep (at which point you can review suggested edits from newer users)
My last piece of advice is easy to say, but not so easy to do: don't get discouraged. The StackExchange network can seem a bit daunting, because we have way more rules and expectations than most other places on the Internet. We do a lot more moderation of content, and we put these barriers (which often seem arbitrary) in between new users and certain activities.
However, keep in mind that we want you to succeed; we want you to stick around and produce high-quality content. If you're willing to work with us, and accept constructive criticism, we're willing to help you polish your content and build your prestige in the community.
1 Unless the question has been protected, but there aren't many of those
Thinking back to my earliest days on the site, I have to say that the quickest way to earn the minimum rep required to begin actively participating is the "unanswered question" list. With even a modicum of research you can usually find and answer a couple of the more niche questions and earn a few upvotes.
The best overall advice I can give is to find a user whose answers (or questions) you like and try to replicate their style and manner of answering. I've found that my own answers seem to be well received because I
always usually obey my own golden rule; If you can't back it up with evidence, don't write it.
This is the most efficient way. For each upvote you get on an answer, you receive 10 points. A decent answer can easily get five upvotes, putting you at 50 reputation.
A good answer is detailed; longer answers tend to be well received, but only if the information is relevant. Quotes and other sources help make a good answer.
You get 5 points per question upvote, so this is a bit slower. On the other hand, writing questions often takes less research effort.
You can look for grammatical and other errors in questions and answers. For each edit that is approved, you receive 2 points. This is the slowest, but there are lots of grammatical errors to correct. There is a minimum number of characters that must be changed, though.
Note also that only 20 reputation is needed to talk in chat, and so if you need help you can get it a bit sooner.
You can also answer most questions, except those that have been "protected" and require a very small amount of reputation.
In general, the tour and help pages (upper right of any page), will give you the info you need to be successful on the site.
As for creating tags, we have a variety of policies in place regarding creating and using them. Even if there were no reputation restriction, you may find that more experienced users, familiar with our policies, would remove tags that didn't fit or add tags that do fit, anyway.
Please see What is reputation? How do I earn (and lose) it? in the help center.
The best way to get your rep up quickly is to provide well thought out questions or answers to existing questions. If your information is correct, there is enough traffic on this site that your reputation could go up a couple hundred points in a day.
I would also suggest you read through how stack exchange works, there are reasons behind needing certain amounts of points to achieve privileges such as creating new tags. The help section is here and the site tour is here.