DD started rolling at 5 months, a bit later than other babies in our group. I didn't do enough tummy time as she spewed all the time. Not that it makes a difference really as she was the first to crawl and cruise the furniture by about 2 months. Its the only 'first' we have with DD - lol.
I found with DD that she could do it intermittently with some 'teasing' with a toy. Mostly, she would roll onto her side and just lay there. It was really frustrating when she started to do it regularly, as she would roll over and crack a wobbly. I think it doesn't hurt to give them some encouragement.
A brilliant book I recommend for you is Dr Mirriam Stoppards 'Teach your child'. As a first time mum also, this is my bible. It breaks down babies development in all fields (mental, moving, hands, socialising etc) monthly. It explains what usually happens and what little activities you can do to help them develop. Its not teaching them to be a genius, just the normal day to day things you can do to help them develop normally.
Of course, its just a guide, but it is the best resource I've ever come across. It makes me feel like I have a purpose when we are playing together. DH really enjoys it too as each month I give him an idea of what activites we need to concentrate on. Here is the example for DD's locomotion section (1 year old):
'Your baby can walk if you hold her hands. When she crawls, she will do so on her hands and feet, in much the same way as a bear does. To help her locomotion you can 1. Encourage independent stepping, practice asking your baby to come to you while she is cruising. 2. Give her the courage to launch herslef off, put your pieces of furniture slightly further apart. 3. Encourage her to walk to you. Sit a little way away from yoru baby shile she is holding onto a piece of furniture. Hold out your arms, call her name, and ask her to come to you'.
Its only a tiny snippit, but thought it might give you an idea of what I was crapping on about - lol.
DD is 3yr 8 months - DS is 6 months