"Really?" - Серьезно? Да ладно?
"Cool!" Круто! Прикольно! Офигенно!
"That's nice" Неплохо!
"Oh dear..." (in reponse to negative news) to be honest we don't use something like this, we just say something like "sorry" without appeal, but you may say "бедняжка", but it's usually used when you tell to your (e.g.) mom that your friend has a problem now and you call her бедняжка
"I see..." Я понимаю, понятно, ясно
Any other used expressions:
Офигеть, с ума сойти - go nuts
Also we often change the word forms like "прикольно - прикольн, прекольна, прекол" asoasf
Какая разница - who cares
Молодец! Well done (only for one person) (we have молодцы for group of people but it's sounds like if you are at school lol)
I did it! - you may say "получилось!" but more often we say "ес" like english yes.
Капец when something bad happened like if you've got some paint on your new t-shirt and you may say "капец мама прибьет меня (my mother will kill me)"
Да ты офигел! (or ?) How dare you?
We have a lot of words that based on the word фиг like офигел (if я офигел! I was surprised, but if ты офигел? means are you kidding me, are you an idiot? When a person from your school poured watter on you on purpose), офигеть (omg), офигенно (nice), иди нафиг (like step the fuck off but not so hard, it often use children) etc
Ору - something like lol or lmao, really useful among teens irl and in social networking
Also omg, irl and lol we may say in real life and in SN
Really useful блин! like dumn, it use aaall people starting with children