Well, first thing is first. Read the rules on claiming, and the forum rules so you know what we expect. We don't expect your work to be Bethesda quality (not saying it won't be anyway), just give it your best, that's all that counts.
Secondly, at the moment, we have no master file with the assets; it is being worked on. For the time being, we are using Silgrad Tower's assets from their esm, but I wouldn't recommend you let your plugin depend on another team's master file, but if it's the only way we can work, then so be it. The plugins can be cleaned and removed of dependany later. So download Silgrad tower's latest modding esm and check out the Silgrad Tower updates. When you have those, you're ready to start modding.
For the landmass, check out the pinned
Vvardenfell Heightmap thread by Lightwave here in the Vvardenfell forums, scroll down to a download link, and download it. Optionally, you may download higher resolution texture pack, making the distant landmass prettier. As far as I recall, the heightmap is not final, and may have a few small errors (because of the LOD generator), but it's stable to use nonetheless.
When you have both, you're fully ready to start. I strongly recommend that you claim
interiors only, we will be making the landmass and the city/town exteriors later, but (depending on the location of the claim) you can mod the exterior, but only very basically. Large areas, like the cities and large exterior locations are the main areas which won't be allowed to be claimed, but small locations such as most dungeon exteriors, small towns/settlemts exteriors etc. may be built in your claim.
Also, please understand that the Silgrad Tower team and TES4 Vvardenfell both may use each other's assets as long as we follow each team's rules.