Portrait Photography
Images of people sell very well, most of the celebrity stock photographers have made their portfolios predominantly from people pictures. People images need to convey an emotion, concept or lifestyle. Someone talking on a phone with a big smile; someone getting frustrated using a computer; children looking bored doing their homework. Faces need not necessarily be shown, some concepts are better defined without the distractions of a face e.g. a walk in the park holding hands where the subjects are walking away from the camera. A model release is needed for all people photos, plus I would recommend getting a release even if just a hand or other body part can be seen. Some agencies treat body parts as potentially recognisable even if they do not have something recognisable like a tattoo on them, so err on the site of caution and try to get a release if you have the opportunity.Tabletop Photography
This is the staple of the stock photographer. You can save such photo sessions for the proverbial rainy day or during unexpected downtime. Food sells especially well, as do computers and technology concepts. Many of these subjects are however very well covered.One trick here is to 'accessorize' the photos to create something with more of a concept. Instead of 'computer keyboard' think 'working from home' or 'overworked in a busy office' and instead of 'bowl of salad' think 'fresh salad eaten out on the terrace'. Constant research (be looking out for photos wherever you go) will help, for example immerse yourself in food magazines and illustrated cookbooks that contain photos in a style you like. People make their entire career out of taking images of specialist subjects so there is a very high standard of work currently available. Table top is probably the easiest place for the beginner to start out taking photos specifically for their stock collection, although it depends on your connections, you might also consider mastering portrait photography and shooting models if you already have some willing volunteers to practice on.
Take care when choosing subjects to photograph, make sure that nothing contains any logos or branding.
Landscapes
If they are to be accepted and sell at all then they really need to say something, even if it's just 'wilderness' or 'farm land'. Landscapes won't work if the subject is too generalized like some fields with a mountain in the distance, even worse a sunset. Almost always the landscape will need some people in it to give it scale and allow the viewer to imagine themselves there. There are lots of photographers who specialize in fine art landscapes, taking photos in just the right light, some of these sell, but it's better if the photo is taken in a "travel photography" style. With just a little extra planning when setting up such landscape photo trips can be used to create some stock images.Travel
This is a popular one with the beginning stock contributor, everyone takes holiday snaps. To sell well the photo must not just depict a location but capture some essence ofwhat it's like to be there. 'Lovers in Paris', 'Snorkeling on the Barrier Reef', 'Snowboarding in the Alps'. Simple travel photos are usually more than just landscapes, landmarks and cityscapes. There are good sales to be had from simple shots of an iconic landmark subject despite the amount that these have already been captured, this is often exactly the " cliché " that some buyers have in mind; that said there are lots of buyers looking for a different aspect on a well-known subject.