Perhaps the two most important aspects to using a digital camera - those aspects you must consider before leaving home - are storage and power. These days you can buy as much memory as you can afford, but what about power?
Types of battery
Digital cameras eat batteries! Forget about alkalines except as an emergency power source. You need rechargeables. For the Nikon 990 Nickel Metal Hydride (NiMH) cells are the way to go. They are available in capacities from around 1200 to 1800 mAH, with the higher capacity units costing slightly more but powering the camera for about 50% longer. The capacity of the battery should be marked on it. If it isn't, perhaps that brand is best avoided! Even the top brands are now relatively cheap. The higher the capacity of a battery, the more charge it can store, so the longer it can run your camera. This may not matter to you, or it may be critical in capturing that perfect shot at the end of a long day.
Recharging the batteries
In addition to batteries, you also need a charger. The cheaper chargers simply dump a fixed amount of current into the battery until it is charged. Because they do not know how charged the battery is already, for safety they tend to use a smaller amount of charging current which results in a longer charging time. Personally, I would avoid this type of charger.
Better are the intelligent chargers like the Maha MH-C204F (available from Nevada Electronics in the UK - see their Accessories section) which determine when the charge has ended by monitoring the battery during the charge. These will charge a set of 4 cells in a couple of hours. Because NiMH batteries dot not suffer from the memory effect which plagued NiCd cells, there is no reason to use the "reconditioning cycle" which does a full discharge followed by a full charge. Of course, that may be a good way to test out some cells about which you are uncertain - perhaps newly purchased or second-hand ones.
Don't stop and restart the charge. During charging the battery voltage will change, and restarting the charge will confuse the charger about the state of the battery. If you have to do this - perhaps you had a short power cut - let the batteries rest for perhaps 15 minutes before restarting the charge. Keep the lid of the charger open whilst charging as the batteries can get quite hot.
If you are travelling, consider what voltage and what mains connector to get. If you travel a lot in Europe, perhaps get a 230V charger with a European mains plug so that you don't need to carry a mains adaptor. You can get a UK plug convertor for home use. For the USA, Canada and Japan, a 110V charger with a 2-pronged, flat pin mains connector is the way to go. You might want to take more than one charger if your access to mains power is limited.
Please be aware that the number of charge cycles these batteries can sustain is limited to about 500 cycles. So if you recharged them every day, they would be dead in about 18 months. So recharge only when required for use! However, also be aware that the charge can decay by up to 50% in a single month (see the Maha product specifications such as the MH-AA160 Product Technical Data Sheet (an Adobe Acrobat file), so the capacity of the battery will be less if it left after charging. So, once again, the message is charge just before use.
With batteries as cheap as they are, you'll probably want to recharge even partially used batteries to make sure you have fully charged batteries at the start of every day.
How many sets of batteries to get
That's really up to you. We find that it's often that we have to change batteries once a day, and sometimes twice a day. Mind you, this is using the LCD viewer a lot as we prefer to edit out the bad shots in the field! Consequently, I now take four sets when travelling - three packed in a box and one set in the camera. Using the higher capacity batteries may mean you need fewer of them, or may not run out of power just at a critical moment.
How to distinguish between the sets
It is important not to mix batteries from different sets, or with different states of charge. What we have done is to buy different sets at different times, or from difference suppliers. We have the original Uniross batteries (green), some NEXcell ones (silver), those that came with the 990 (blue) and some recent Powerex 1600 mAH units (orange). Cecilia discovered that you can store up to three sets of batteries in an old Kodak yellow slide box! Put the used ones back in upside-down like the orange ones below.
To learn more about Batteries
.. you could try the Maha Web site. (No this isn't Maha sponsored, their products happen to be what I've used and been happy with!).