Before I really get into this article, it's important to note that you do not need a Steampunk persona (or "steamsona") in order to be a Steampunk, or wear Steampunk clothes. Some people like to pretend to be a different person when dressed up in Steampunk clothing, but that is entirely optional, and only to be done if it appeals to you. That said, even if you aren't actually acting as your character, it may still be helpful to have one in mind when putting together an outfit. Personally, I've found that it really helps with outfit cohesion.
With that disclaimer out of the way, let's dive right into it.
Steampunk personas should really resonate with you in some way, or else you'll find that you may start to run short on inspiration, or get tired of working on it before you finish. Steampunk, along with costuming of most varieties, is a labor of love. If you don't love it, you'll just end up hating yourself.
So how do you choose a persona that resonates with you? I can't really say exactly how, but usually you know it when you see it. I'll list a few typical archetypes and then give you the tools to create your own. In Steampunk, most characters are based around what they do, so that's what we'll look at.
Pretty much everyone knows what a pirate is, and in Steampunk the pirate is used most often aboard an airship, hence the common trope of the airship pirate. Airship pirates usually have outfits that have been cobbled together from things they've "picked up" (aka "stolen") here and there, and their personalities range from the good-hearted rogue to the malicious scoundrel.
Photo by Anima89
Likely gadgets for a pirate would be things like goggles, vision-enhancing things (telescope, periscope), navigation things (compass, sextant), short guns, and other small hand-to-hand weapons (despite the huge hammer in the above picture).
An adventurer is generally a person of means who has decided to go "adventuring". Their outfits tend to be more coordinated than the pirate, and may be more reminiscent of formalwear. Gentlemen may wear a tie or cravat, and women may wear a corset and/or bustle with a shorter skirt.
Adventurers usually have a spirit of, well, adventure, and want to explore the world and get into trouble. Typical gadgets for an adventurer would be a weapon of any variety, some type of body armor, pouches for holding things, communication devices, and maybe a cane or parasol.
A scientist is someone who Does Science in capital letters. Their outfit may be reminiscent of a lab coat, and may also include goggles, gloves, glasses, and other words that start with 'g'.
Photo of/by Rob Flickenger
Scientists may lack social skills, and like to pursue knowledge through experimentation.
Photo from John C. Wright
They can be the typical "mad scientist", or the more sober research scientist. Gadgets that a scientist might have include things like vision-enhancing microscopes, Tesla coils (like the Tesla gun above), recording equipment, safety equipment, etc.
Aristocrats generally live a life of leisure and can do whatever they like. As such, they can either be selfish or philanthropic, mean or nice. Their clothes are Nice with a capital N, and are always formal. For the ladies, fancy corsets (over or under the clothes, depending), lace, and hats that are either too big or too small.
Photo from John C. Wright
For the gentlemen, coats and vests are required, as are nice shoes (never neglect the shoes!).
Photo of/by Kit Stølen
Above all, aristocratic outfits are either very tasteful, or are very deliberately over-the-top. The middle ground will just make them look sloppy. Aristocrats may not even carry gadgets, because they don't need them. However, they can wear goggles, jewelry (for men and women), hats, spats, and other refined things.
Ragamuffins have dirty jobs, and are in many ways characterized by the dirt or soot that's all over them and colors their clothes. They may not even have a job at all, and might be street urchins, beggars, and the like. Their clothes are often either patchwork or scavenged, as are their gadgets.
Photo of/by Crackitus Potts
This is one of the few types of characters that can look disjointed in terms of outfits, because the dirtiness will tie everything together, visually. These characters can literally wear or carry anything, so long as it looks used and broken-down!
The above examples are just that: examples. You can see more here, but there are many variations of the above, and the most important thing to remember is that you can pick and choose, and make your own. I'd start with a profession, like mechanic, doctor, miner, captain, spy, factory worker, or even odd things like banker, blacksmith, janitor, or mortician, and then go from there. Think of the tools that someone of your chosen profession might use, and then think of ways to Steampunk them. Alternately, think of tools they don't use, but which might make their jobs easier.
Your persona's specific personality is largely up to you, as pretty much any profession can encompass people of a variety of alignments. If you really need help coming up with personality traits, I suggest you check out this list of NPC traits, or even this trait generator (refresh the page for new traits). This PDF even includes a list of physical traits for you to play around with. Obviously you can't easily make yourself taller or shorter, but you can wear wigs to change your hair length, and use makeup to make yourself appear older, younger, thinner, fatter, etc.
I'll be honest, most Steampunks don't bother to change their inherent physical appearance for the sake of their characters, but some do! If you really want to try being an entirely different person, by all means, go ahead!
Having a Steampunk persona is about creating a complete, cohesive picture of a different person. Try to be consistent, too, without dropping in and out of character too often. Think about what your character would say or do in certain circumstances, and then consider why they would say or do that thing.
It may sound complicated, but once you learn how to do it, it can easily become second-nature. I recommend starting small and "feeling out" a character before you start making huge time and money investments in clothing and props, because you may find that certain characters just don't "flow" naturally from you, and it can make you lose interest in being them.
Above all, it should be fun! Having a good time is the most important goal!
If you have any questions, feel free to ask them in the comments section!