Hi Sophie,
First things first, well done for starting now! Most people, myself included, don't start thinking about this sort of thing until much later in life.
Couple of things to think about.
Cash Savings cushion - It is always good to have a cash fund available in case of emergency. This should really be the first thing to think about. Open a mini cash ISA with the best interest rate you can find and start putting your £100 per month away. You should aim to have 3-6 months salary for those 'just-in-case' situations. Saving in an ISA will protect your interest from tax which is always advisable when possible.
Longer term savings - These can be a mixture of things...
You may want to start investing in your pension. I know it is boring and a long, long way away, but the early years really do make a difference. If your employer matches any of your contributions it would be worth making the most of it.
Investing in the stockmarket. This is higher risk than cash but over the long term, which being only 20(ish) you can look at, it will beat cash returns. You might be able to invest through company share save schemes, which offer attractive returns and a cheaper way into stockmarket investing.
If I were you though, I'd concentrate on building up a cash fund. The rest can come later once you have this safety net in place.
Hope this helps...