The biggest thing you must do is make sure you get a properly fitted bike. Doesn't matter what type of bike road, mountain, cross, or whatever get fitted and it will make your riding that much more enjoyable.
If you haven't got a bike yet and you have a shop nearby take several different brands out for a test ride. Also realize that people's seatbone's are different and saddles can be changed out. As far as clothing a good pair of bib shorts are the way to go . Check to see if a shop has a beginner's group ride, and join in and ask question and watch others and learn. It is absolutely a great sport to get into and can be very addicting, you always want to get better or beat the time of your last ride, or rip up the bigger hill, it never ends. But most importantly just get out their and enjoy the freedom of the open road..