One of the problems in defining Steampunk is that as soon as you start describing the genre, you start creating boundaries. While a general definition is necessary to get people going in the right direction, do not be overly concerned about crossing some unwritten rule, as they largely don't exist.
Steampunk might be broken down into three components: history, science, and creativity. Many of us became attracted to steampunk precisely because there is so much flexibility in the style. It's a young and ever-changing genre. People are constantly generating new ideas, and we embrace those ideas.
So be creative. Don't hold back an idea out of fear that it won't fit some narrow and spefic definition, because that's not generally how we work.
And that's not just about costuming. The Steampunk community expresses itself through visual arts, music, storytelling, music, comedy, engineering and so-forth.. We're all about self-expression, creativity, imagination and innovation.
Steampunk is traditionally set in the Victorian time period. That does not mean we have "costume Nazis" or "period police" pointing out historical inaccuracies. First, we're only vaguely Victorian (sub-genres step out of the time period), and, second, we're science fiction. This isn't the Victorian Age of 100 year ago...it's the Victorian Age on some alternative Earth.
Victorian fashion is a good place to start, but it doesn't have to be your end point. There's a lot of anachronisms in steampunk. We even let our women wear pants.
Many people, particularly when first building a costume, are concerned that they don't "look steampunk." As you should quickly see on this site, basic Steampunk costuming is actually very simple. Not every steampunker is a mad scientist with fifteen gadgets hanging off him. For the basics, check out:
Once you've got some kind of base, adding a couple accessories can quickly make a costume look more specifically steampunk than simply historical. Check out Accessories for Men and Women.
Again, however, absolutely none of these are necessary. Do you look like someone who might live in an alternative Victorian Age, whether as a worker, aristocrat, professional, soldier, mechanic, explorer, pilot performer, scientist, etc? Then you're steampunk enough.
There are places in this website where I point out certain no-nos, such as tennis shoes. If you're trying for a complete look, you should have something other than tennis shoes (and I give plenty of suggestions).
That doesn't mean we're going to point and laugh if you show up with tennis shoes on. This is an inclusive community, not exclusive. We have a wide range of ages in a wide range of economic situations. No one is expecting you to drop a bunch of money into this hobby. If tennis shoes are all you own, so be it. It's not the end of the world. But there really can be some cheap alternatives (some of which may already be in your closet).