Cycling doesn't target JUST the butt though...it's a fairly compound exercise on your legs, but isolation-ist in that most of your results will come from your quads. Calves more than usual if you use clipless.
I've ridden bikes my entire life, and at one point owned 3 different styles of mountain bikes (street, Trials and XC), 2 different styles of road bikes (road and fixed gear), and 3 different BMX bikes (race, street and flatland) AT THE SAME TIME. I can tell you after a day of even riding over bridges multiple times with many inclines along the way, you won't get quite the butt you seem to be after (the workout wise), unless you ride for HOURS a day, as in a MINIMUM of 4 if you're just road biking. That might seem like a long time, but cruising along at 15mph (more often than not, you'll be going like 10-12), that's only ~40 miles....by road biking standards, that's pretty weak.
Nothing like old fashioned and boring lunges. Trust me, after doing 1/8th a mile of long, rear straight leg lunges...your glutes will be one fire. I've done 220 miles on a roadbike in a matter of hours many times, and I'll tell you, my butt wasn't even TOUCHING the kind of soreness and workout I gave my glutes with 1/16th-1/8th a mile of tip-toe lunges. I'm still massively sore 2 days after that workout in my glutes.
But if you're SET on getting into cycling, do yourself a favor and get a road bike with nothing less than Shimano 105 components. Anything less than that, and you may as well be on a walmart bike, because that's about where the quality of anything below 105 sits. 105 shifts better, is lighter, stays in true/tune longer and just outright is better. Scour craigslist for some deals, eBay works, but if you want brand new bikesdirect.com is the best place. I use to work directly under the OWNER, and as much as I hate to say it, the best deals you'll ever find are on there....bar none. You'll get better prices on BRAND NEW BIKES of higher spec than some used bikes found elsewhere.
