Congratulations!!!! I wish you the happiest, healthiest pregnancy. And of course, baby too!
You can still continue to exercise throughout the pregnancy. You may need to alter the intensity as you grow, but for right now, even light aerobic exercise is okay. There are some people out there who water-ski, rock climb and so forth. I was never one of those people, but I did go to Lamaze classes and did trimester appropriate exercises. Walking is ALWAYS acceptable. I had toxemia aka pre-eclampsia with the first two, which cause MASSIVE weight gain and couldn't do much of anything toward the end. I enjoyed my 3rd much more with a gain of about 30 lbs.
I think as far as food goes, eat when you are hungry but definitely stick to more healthy, whole foods. Try and stay away from sodium as much as you can. This doesn't mean X it out altogether, but you will see a lot more swelling during the end of the second and all throughout the 3rd trimester if it is not kept in check.
These things will all help you maintain, and hopefully gain only the recommended 20-30 lbs. Some gain more, some gain less. I was always told by my GYN that no less than 15 and no more than 40 is a sign of a very healthy pregnancy.
I never did this with any of mine, but I have read that slathering yourself up with cocoa butter everday several times a day helps with the stretchmarks and prevents the loose skin. I don't know if I actually believe it. I think it depends on how much you gain and in my case, the weight gain could not be avoided with the first 2, and also depends on genes in general. But some people swear by it.
Most of all, just enjoy this time! Again, congratulations