Navek, just like any exercise, its hard when you begin, and as you go, it gets easier. To me, the most important thing is comfort. If you are not comfortable, you wont be able to enjoy the ride and struggle. I suggest you visit your local bike shop, get fitted (figure out what size bike you need) and ride a few bikes in your price range. Focus on comfort, not brand names and shiny colors. Most common complaint is the seat/saddle discomfort. Your butt is going to feel uncomfortable at first, even if you have a Brooks (leather saddles are my favorite) saddle, but with time, you will get used to it. Your commute isn't very long, you will be OK. Extra gel seats are useless. I recommend padded shorts or pants for colder days. Second problem could be some pain in your hands. A good handlebar tape and maybe some padded gloves will work great. I recommend cork tape or leather (synthetic tape does not absorb moisture makes things worse on longer rides). Last but not least is the pedals. Ride a bike with platform pedals and then try toe clips. See how they feel. I got used to toe clips but ended up getting clip-less pedals for speed and longer rides. Comfort is the key. There are a lot of helpful people at local bike shops that will be happy to help you with your questions and might suggest something you might not be advised on here. Everyone is different, what works for me might not work for you.
Your commute is not very long if its 12 miles round trip. It wont be easy when you begin, but trust me, more you ride, easier it will get. Just not the first five to ten minutes while you warm up