Tinkerbell - congratulations, try to look after yourself and keep fit (walking, swimming etc) & it will help you towards then end of your pregnancy when you look like Mr Greedy!
If your twins are sharing a placenta, then obviously you have a higher risk pregnancy but you will be monitored frequently and you will find out soon enough after the birth whether or not they are identical.
I have twin boys who are now 2 years & 3 months (and a 4 year old daughter) & they are great fun as well as the old cliche double trouble! I didn't find out I was having twins until 21 weeks so it was the biggest shock of my life as there is no history! I found the first two months incredibly tiring and stressful until I got them into a Gina Ford type routine. I recommend you get as much help as possible in the early weeks and join your local twins club (TAMBA will give you details or look at twinsclub.co.uk) & they will put you into touch with other twin mothers and other people expecting twins so you can go through similar experiences together - I didn't get this opportunity and I wished I had. The Multiple Births Foundation runs an information evening at Hammersmith Hospital at which there is a question & answer session with parents from a local twins club (they may run similar sessions in other areas). There is a group of mothers in our local twins club who were completely sorted by the time their twins were born and they have coped brilliantly with seemingly little stress.
I had an elective caesarean at 38 weeks (6lb 4 & 6lb 11) as the first twin was breech (after having had a natural, epidural-assisted, birth with my daughter) & that was absolutely fine although you do take longer to recover.
If you want to breastfeed, make sure you get plenty of support in hospital. I was put into a single room, which was great for privacy, but the call button didn't work so I was left to fend for myself when trying to breastfeed. I found it better to breastfeed them together but I introduced bottles early so that other people could help feed if they were around and the twins were happy with whatever they were given - breast or bottle!
Good luck; try and enjoy some quality time with your older child while you can and take care of yourself.