Hi. It will depend on what your interests are and what activities might be important. For a first cruise, I would say you can't beat Royal Caribbean. They are not cheap but have some deals on at the moment. They are principally an American line so service and food are a notch above some other lines. Plenty of activities onboard for all ages, kids clubs etc. I would not recommend sailing out of Ireland as you are likely to encounter some swell on the seas until you reach the Med, if that's where you are going. The Baltic and Norway cruises are popular that time of year but more expensive. Expect lots of kids onboard, as it's summer hols. If you can travel outside of secondary school hols, it would be cheaper and less busy with the little ones. For an idea of the costs and itineraries, I use
www.vacationstogo.com, a US travel agent. They are not always the cheapest but they have a comprehensive database of cruises in all regions, another is
www.seascanner.com, a German travel agent, again not always cheapest. I have booked twice with vacationstogo as they can get you US prices which are sometimes much lower than Irish prices.
Here is a link to VTG.com showing Royal Caribbean cruises in the Med for July/August 2018.
http://vacationstogo.com/ticker.cfm?sort=v&r=11&l=14&sm=20187&tm=20188&n=2
Symphony of the Seas is a new ship which will be packed with feature to keep all ages busy.
There is a Holiday world show coming up in Dublin and probably other areas too. Good for an idea of the options. Perhaps use an Irish travel agent for your first cruise? As to when to book, the usual payment date is 90 days before sailing. It is after this date that price drops may occur. If you book earlier and prices drop after that, you cannot get the lower price although you may be able to haggle some onboard credit. It's also a gamble as a ship may be sold out and you may not get the stateroom you would have preferred if you leave it last minute. There area a few itineraries in the Med, Western and Eastern principally. Barcelona is a convenient starting point as there are lots of flights from Ireland, similarly Venice although the larger ships do not sail out of there now but it's convenient for Irish travellers as there are two airports and the city is close to the airport. Bear in mind also, that, all cruiselines add on 'Daily Service Charges' or 'Tips' as we used to call them, to your onboard account, usually about €10 pppd, including for your teens(€280 for a 7 day cruise). Also, drinks can be expensive so you should explore the all inclusive options. They will usually save you money, depending on how much you like to drink! Cruise lines make a lot of money from Shore excursions. You can usually arrange the same type of trip locally with some research on the internet but you may worry about missing the ship although the firms that do them are really good at getting you back on time. Most European ports can be done DIY without much hassle.
PM me if you want any specifics of our 10 cruises(8 done and 2 to go). Slim