There are a lot of gi companies out there, and all of them want your business, and that's fair. Just like any other product, the manufacturer wants to make their product more appealing, but also recognize that they're in a niche market with a specific demographic. Some gi companies stay more conservative and traditional by specializing in traditional white gis. While others like to add more flavor to their product by putting them out in various colors, perhaps with patches, and even with built in rash guards. When you're considering a gi (and probably your first) you should consider your gym's rules first. It's not unusual for some gyms to require a more traditional approach--only allowing white, and sometimes blue as well--while other gyms may be a little more loose on their gi policy. I only mention it because it's an easy oversight and you don't want to be the new kid who shows up to a traditional school with a $200 USD+ black Shoyoroll that you can't even wear.
Once you've determined what your gym allows, you're next step is where to buy a gi. Some gyms sell them on-site, but if you're not lucky enough for that and don't have a Gis-R-Us in your neighborhood, the internet is your best friend and enemy. It's your best friend because it's basically the only place that you can shop, but it's also your enemy because you can't look at your perspective purchase in person to make sure the fit is perfect. (SPOILER: The perfect fit is rare.)
The first thing you need to consider in buying a new gi is the price. Don't go shopping out of your budget. A Brazilian Jiu Jitsu gi can be an expensive venture, but it is an investment; and like most things, the more you invest, the better the product, and the bigger the risk. You're new to Jiu Jitsu, that means you're not certain you're going to love it forever and ever, so it's dumb for you to spend a lot on a flashy new gi. It could be a waste of money if you end up quitting after a few months, but more importantly, you'll look like an asshole if you show up in the flashy new gi as a new kid to the sport. Others might downplay that a little, but it's true, you'll be 'that guy' and in the process, look like an asshole. All BJJ gis are essentially the same, though. I say that because you can get a good/solid gi for under $100 USD that will work just as well as the expensive ones--they might not last as long, but they work just fine and are a safe, calculated investment. One of the best things on the internet for this process is http://www.bjjhq.com/. BJJHQ sells 1 BJJ-related item a day at a discounted rate; and usually have at least a gi a week on there. If you haven't already been to the website, get ready for your new addiction. You'll find a lot of great stuff on that website (everything from mouthpieces, cups, and rash guards, to gis, DVDs, and no gi shorts), but it's a great website to find a good gi for sale at a nicely discounted price.
Since we're all not the same size of human, gis come in different sizes. Assuming you're an adult, your sizing will start in the category "A1." "A" meaning "Adult" and "1" meaning the smallest adult. As the number goes higher, the gi increases in size. Most (if not all) manufacturers have a size chart associated with their product. In my experience, all of the charts measure differently. By that I mean that some measure across the chest/shoulders, while others take that measurement vertically (as seen below).
Confusing, right? Yeah. It's a real trial and error process. Generally speaking though, if you eyeball the size chart and order what you think will fit best, it probably will. You might swim in it a little, but that's okay--it's better than it being too small. (Pro Tip: Pay attention to what others your size are wearing and ask them what they like/don't like about their gi, and keep mental notes). If you're especially tall, or short, for your weight, you're probably going to bump into some issues but it shouldn't completely hinder your time on the mat. Remember when I said that the gi companies pander to a niche community? Use that to your advantage--feel free to reach out to them via email or a phone call to ask questions. They want, NAY!, need your business, so it's in their best interest to provide good customer service to you.
'But what about weaves? What's with all this weave business?' Weaves are the type of stitching the manufacturer uses to make the gi top strong(er). Dependent upon what weave your top is, can determine its durability over time, strength, its level of shrinkage, and so it. The world of weaves is intricate, and Fenom Kimonos summed it up perfectly here so I won't try to rephrase what they've already done perfectly.
In closing, you're going to need a gi. Get something that abides by your gym's rules, isn't super-flashy, and makes sense for you and your size. It's a lot to digest, but it ain't nothin' but a gi thang.