We had a wonderful trip to Rome a few years ago and managed to cram an awful lot into four days! (but not quite enough time to go out to the catacombs which are a bit further out). As always "The Rough Guide" was our main guide - if you can get a copy from the library then it gives masses of useful information, including the practicalities of getting around and also local customs - like the practice in Italy of charging extra if you sit down at a cafe table rather than standing up at the counter....
It is also worth visiting tourist websites which give a lot of up-to-date information like prices and hours of opening, as well as useful information about where to go.
The Forum and Colosseum are a must - close together so allow a session for that. The Vatican always has a massive queue (bearing in mind that there are a load of pre-booked groups who get entry preference) so you need to be there either at the crack of dawn (at least an hour before it opens) or risk going later in the afternoon, by when the crowds have considerably reduced. They do have a useful sign system giving you an estimated waiting time from that point in the queue! We went in the afternoon, but found that we hadn't allowed time to see everything before it closed. In particular I was disappointed to find that the main art gallery is actually in a separate block (in the first courtyard you come in by) and had closed when we got there. If you are interested in art, it would be best to do that first before joining the main tour route. You also have to expect a long queue to get into St Peters - which is one of the reasons we did it that way round: St Peters in the morning and the Vatican in the afternoon. But it is worth it!
If you are indeed interested in art, then I recommend the Galleria Borghese, which is in a lovely park (worth a wander round anyway) and has some superb art and sculpture. Another place with some splendid art is the Church of Santa Maria sopra Minerva ( a wonderful example of the Roman Christians taking over ancient pagan sites!) - a hidden gem. Another example of Christian takeover is of course the Pantheon - a fantastic building not to be missed.
Ice cream.... the Rough Guide recommends "Il Gelato di San Crispino" (via della Panetteria 42) just round the corner from the Trevi Fountain - and I would endorse that

, although there are plenty of places as you go around.
Eating out: we were staying at a cheap hotel out in the suburbs and were delighted to find that nearby was a wonderfully cheap restaurant where we could eat well (used by the locals, which is always a recommendation). A predominant feature of Roman food is the use of lemon - often as a simple dressing (without the oil and vinegar) and certainly as a feature of the dishes on offer. This suited me well as I love it, but is something to note if you are not used to it.
The other thing that really surprised me in Rome is that people were very reluctant to speak English, unlike other major tourist destinations in Europe). Not sure why this was (I would expect it in France rather than Italy). It would really help to have a smattering of Italian (which is not so hard if you ever studied Latin and French) - so a phrase book is recommended!
The length of this post is an indication of how much I loved Rome - and am grateful for the opportunity of that brief visit...
Have a great time
