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A beginner would do well to get a large flat tarp, in the 9'-10' on the longest side range. Square tarps will let you do a diamond pitch, or any other pitch. I think a basic A-frame pitch would be the most common, but the nice thing about a flat tarp is that you have options. You can experiment with a cheap blue Walmart tarp, but a silnylon tarp, made specifically for ultralight backpacking, will be easier to pack, lighter, easier to pitch and generally more durable. Check out Oware to look for a basic flat silnylon tarp.
A catenary ridgeline tarp or "cat tarp" (pictured) provides some advantage in your ability to get the tarp taut quickly in an A-frame configuration without additional pull outs. Flat tarps sag and will require additional guy lines. Since this is a very common and useful pitch, it tends to work out, although you can be limited in the ability to really buckle them down in a storm. There is also a cost of losing head space and pitch options with a cat tarp.
My preference right now is a closed shaped tarp - a pyramid-like shelter design (will post picture in next post). It provides a lot of stormproofing, the most head room of any design and all the floorspace is going to be protected from the elements. The disadvantage is losing some airiness and your views of your campsite through the night. It can be really nice to be laying under your rain shelter and be able to star gaze until you go to sleep; you can't really do that with a pyramid tarp.
There are also open shaped tarps, like the MLD Patrol Tarp Shelter or Zpacks Hexamid Pocket Tarp (many ultralight companies make similar sorts of rigs), which are shaped, usually with a catenary ridgeline or half-pyramid type design for ease of set up, but provide significantly more stormproofing than a typical cat tarp. They are also more open and airy than a pyramid (usually no zippered door), so stargazing while you're in bed is possible.
