One thing to consider is whether you wish to sail from UK or fly to the Med to meet the ship. If you fly, your baggage allowance may be quite restrictive, but if you sail from UK, you can usually take as much luggage as you wish. Do you prefer formal type cruising (dressing up for dinner etc) or more casual, freestyle holiday? Tipping is included in the price with some operators (such as Thomson), and discretionary with others (P&O etc), but some lines automatically deduct a set amount from your account each day, so allow for it in your budget.
I agree with the comments from Mr/Ms Smith but you might also consider the age group you will be joining. Generally families choose larger ships and couples/over 50s choose smaller ships that are more friendly and often less crowded. Also consider whether your ship can dock in most ports or if you'll be taken ashore by tender. This need not be a problem, except that waiting times for tenders can be quite long. When you consider all the decisions you need to make when booking a cruise it's amazing any of us ever bother - but we do, year after year - be warned, once you've got 'the cruise bug' that's it! Judith.
If I can add a further piece of advice. When you've made your decisions based on the good advice given by Smith & Cooke don't go down the High St Travel Shop, speak to a Cruise Specialist who knows cruises & can advise you on a cruise to suit your needs. A good starting point would be the Freephone number on the top right of this page. There have been many tales of woe on this site from people who have been sold the wrong cruise through lack of knowledge/skill by some travel agents.