I am making fajitas as we speak -- I am poaching chicken thighs in water with salt, cumin and chili, then I will "pull" it like pork. I also have a pan of fried green pepper and onions. I will serve with grated cheese, sour cream, guacamole and salsa on flour tortillas. Remus has good recipes. Black olives would be a good addition, but I don't have any. I guess it's a bit like tapas in that you set out the bowls with the various fillings but you use them to make your fajita, rather than eating them separately.
If it's your first time making Mexican food, I would purchase flour tortillas rather than trying to make them yourself. (I would buy wheat, not corn, because you can heat them in the microwave with a damp paper towel the corn ones are better heated on the stove or used in a bake like enchiladas.) These are different from Spanish egg tortillas they are just a thin flatbread.
Mexican is great for vegetarians because there are so many beans. I cook a lot, but I do still buy "refried" beans in a can -- if your supermarket has a Mexican section, they might have that. There are recipes online, too. Then you just make your fajita with beans instead of meat.
Something that isn't really Mexican but quite delicious is to put a can of plain kidney beans or black beans in a frying pan on the stove, with some but not all of the liquid from the can. Heat slowly, crushing the beans with the back of a wooden spoon, so they get a little pasty as the liquid evaporates. Add a thick layer of melted cheese. Scoop out with tortilla chips. Could add sour cream and salsa.
You could also cook a rice and beans dish as a side -- they really aren't hard and don't require specialty ingredients.