As Jagmanx said, the first question is whether you're looking for a good way to tap free wifi signals or plan on getting online in the van with a sim. There are some amazing bits of kit out there to pick up free wifi from a greater distance if you want to go that route but I think most people find it easier to go down the route of using the mobile data on a sim.
I've got a few ways to get online. First, I can connect with my phone and can then share that connection as a wifi hotspot for other devices. However, my data allowance is only 1GB a month, so I tend not to do that for general internet browsing. I've also got a cheap sim in my ipad but again with limited data, which is fine for when I'm out and about at home but not enough for trips away in the van. Mostly for camping, I prefer to use a mi-fi unit, like this one...
HUAWEI E5330 White 3G SIM Free Mobile WiFi UK: Amazon.co.uk: Electronics
This one is unlocked, which means you can put a sim in there from any mobile network... there are lots on offer - just look for a '
data sim' because you don't want or need phone calls or texts on it. Once this mi-fi unit is connected to the internet, it sends out a wifi signal, so you can share the connection with up to 5 devices at a time. This is especially good if there are 2 of you and you have separate phones or tablets, laptops etc. My husband and I used to connect both phones, both tablets and a laptop to ours

I've found this is petty much all I've ever needed.
As for the sim... a lot of people buy the Trio sim from 3 that has 12GB of data on it that won't expire for a year, so you've got plenty of time to use it up. Lots of sims are monthly deals... use it within a month or lose it! It's up to what would suit you best as to what sim you go for. I've found pretty good coverage around the country, so I manage to get connected more often than not... plus you can use this sim abroad without incurring extra roaming charges.
OK, so once you've chosen a sim from a mobile company, now you might worry that the signal for that company isn't particularly good where you are. This is why some people buy an antenna/aerial to help get a better reception from a mobile mast (not the same thing as the wifi aerial I mentioned earlier). If you think you want to go down this route, you'll need a mi-fi unit that has a socket for you to plug an aerial into. Huawei do these too but they cost more to buy than the one I linked to.
Hope this is enlightening rather than confusing!! :rolleyes2: