Yes, this is mostly a costuming blog, but different cons place very different demands on people in costume. So the first thing you need to figure out is: Where the hell am I going?
Now, if you've already got your four-day pass for San Diego Comic-Con (haha, just kidding) and you just need to decide on a costume, you can skip this one. If, however, you're looking for a place to bond with your geeky brethren, the three things you should consider are location, cost, and tone.
The most important question to keep in mind as far as location is: How far are you willing to go?
If you're fortunate enough to live in or near a major city, chances are there's at least one con happening annually in your backyard. On the other hand, if you live a little farther from civilization, some research and planning will be necessary. The good news is that, no matter where you live, the nearest urban center probably hosts a con of some kind. For instance, I live near Memphis, TN, which hosts MidSouthCon. The other news (not necessarily bad, just something to consider) is that a lot of these cons are smaller, which means fewer events, less prominent guests, and a whole lot of downtime (unless you're a gamer).
If you're not sure what's close by, the best thing to do is run a search and see what comes up. Once you've found a con that looks promising, find out what other people (on the internet or *gasp* in real life) have to say about it. A con might have a great webpage but not actually be somewhere you want to be. For another real-world example: CoastCon, in Biloxi, MS, is near by, but I've heard too many horror stories about creepers and harassment to feel comfortable going.
Establish your radius, find out what your options are, and research the hell out of them.
The big question here is obvious: How much are you willing to spend?
Ultimately, your con budget is going to break down into a few basic expenses: the con badge, travel, lodging, food, and swag. We'll talk about budgeting for food and swag in a later entry, since those are generally post-registration concerns. Cost for con passes varies wildly, and you can pay anywhere from $20 to $300, depending on where you're going and how many days you sign up for. Smaller cons are usually cheaper where big cons typically cost more, which is kind of intuitive, but there you go.
Likewise, hotels in, say, San Diego are going to be hella more expensive than hotels in, say, Memphis. Most cons have sponsored hotels that are in the immediate area and sometimes offer discounts for con-goers. The downside to this is that cons are typically held in the part of town that has the most expensive hotels to begin with. Depending on the public transit situation in the city in question, you can stay farther out an commute in, which has its own expenses and will definitely get you and your costume some strange looks.
You should also factor in travel expenses and whether you'll be flying or driving, which comes back to the question of how far you're willing to travel. Also remember that, if your income is limited, more money spent on getting to the con means less to spend on costuming.
Establish your budget, find out what your options are, and do your research.
The question: What kind of geek are you?
Though is a slightly less pressing concern than those of location and cost, it's still something you should consider and will probably have the greatest impact on your costuming choices. Even ff you're severely restricted by mobility and income and can only make it to whatever con is closest and/or cheapest, you should still keep the tone of the con in mind when choosing and designing your costume.
See, while all cons contain high levels of geekery, the kind of geekery can vary wildly. Some cons are geared specifically toward certain types of fans. Otakon in Baltimore, MD, for instance, caters to fans of anime and other Asian ephemera while Yaoi-Con in San Francisco, CA is for -you guessed it- yaoi fans (If you don't know what that means, I'm not explaining it.) and Gen Con focuses on gaming.
Other cons, while not as specific, have their own pace and feel and will certainly appeal more to certain types of fans than others. A major example of this is the difference between SDCC and Dragon*Con, which we'll get into in a later post. In any case, research is your friend. Look at the programming and guest info on the con website. If there are a lot of panels about Tolkien and fantasy fiction, there will probably be a lot of fantasy readers hanging around. If the programming seems to focus on SF television and movies, you're more likely to run into an SG-1 gate team. If Leonard Nimoy is on the guest list... well, you get the picture.
You should also think about what size con you want to go to. If you want to go to lots of panels and see celebrities, check out the bigger cons. If you want to hang out with other fans and have lots of time to chill out, stay small. If you're looking for a combination, hit somewhere in between. This isn't a hard and fast rule, but it's a decent frame of reference.
Know your geek, and do your research.
Deciding which of these three factors is the most important to you will go a long way toward helping you find the con for you. Once you've made you choice (and booked your hotel. Seriously, don't wait on this.), then the really hard part begins. Now it's time to make what is ultimately the most crucial question of your con experience: What the fuck do I wear?
Gods help you.