Since you want to have the pump running 24 hours a day, you need to choose a panel that can output enough power during daylight to power the pump all the time. It might surprise you how big the panel will need to be.
1) Analyse the pump requirements. The power use is 6W. Multiply 6W by 24 hours and you get 144 Watt-hours.
2) Choose a battery that will store enough power. It's wise to over-spec the battery so that it can provide a few days (or even weeks) of power in the case of bad weather. At standard lead-acid battery is a good solution. A lead-acid battery is considered a "12V nominal" battery, although they like to be charged to a higher voltage (13.2V to 13.8V is common).
Battery capacity is measured in Amp-hours. So, for one day, we have 144 W-h divided by 12V, which gives 12 A-h. Choose how many days you want in reserve, and select you battery accordingly. This site is a good reference of what's available, and they also have low prices on the batteries they sell. If you want a 3-day buffer, for example, you would want a 36 A-h battery.
Also, lead-acid batteries don't like to be deeply discharged. If you let the battery drain to almost empty, then the energy storage capacity will quickly diminish. This is why automobile batteries get ruined if you leave your headlights on for a long time. A rule of thumb is to buy twice the capacity you need, so the battery won't become more than 50% discharged. (Thanks, Filek!)
3) Decide if you want to buy a solar charge controller. It'll cost extra money, but it really stretches the power you get from a given panel. With a good controller, you may get 90-95% of the panel's rated output into your battery. If you just hook up the solar panel to the battery, you probably will average only around 40-60% over the course of the day.
4) Calc your required solar panel. There are a lot of options here, depending on what you want. At one end of the spectrum, you wouldn't need solar, because you could charge the battery regularly. On the other end, with a lot of solar cell output, you could plug in a discharged battery and have it charge up rapidly (even with the pump running!).
To provide enough power for the pump to run 24/7, you would need 144 W-h per day from your panel. Assume you get 6 hours of good, high sun per day. 144 W-h divided by 6 hr = 24W. Adding your inefficiencies, that would be a 48W panel without a charge controller, or 26W with one.
Of course, after a few days of bad weather, you want extra power to top off the battery again! So get a slightly bigger panel than what you calculate. I personally would choose 20-30% bigger.
Here is a handy chart that shows how many "sun-hours" you can expect per day depending on your location. If you want more precise numbers, you can do a web search for "solar insolation". (yes, that's spelled correctly!)