taking large pictures
First, shoot RAW files. JPGs are a handicap - the file compression is achieved by systematically throwing away data. And 12 mpix is plenty.
In Photoshop or equivalent, if you need to increase the size of the image to get the print you want, you can use a dedicated scaling program or do it yourself manually. If you do it yourself, use multiple iterations of increasing the size by using percent instead of pixels or inches, and increase no more than 10% in each iteration. Be sure that resampling is turned on. It's a little tedious, but only a little, and it will do an excellent job.
Put the camera on a decent tripod (doesn't have to be a world beater), and use a remote to fire the camera or the camera's self timer - low ISO with hands off the camera is best. f8 is typically the sharpest aperture.
Use accurate focus - make sure you focus on the principle subject.
All that works very well for landscape. Now let's talk about wildlife. RAW is still the best file, and a tripod is still the best approach. With wildlife you'll be using a longer lens, so a higher shutter speed is necessary. Forget about the self timer or remote; animals move, unlike the landscape (you can quote me on that). Shooting with a wide open aperture is common, but you'll probably need higher ISO, maybe even into the 1600 or 3200 range, depending on the light. Animals tend to be out feeding in the hours around sunrise and sunset - great light, but not a lot of it. Accurate focus is crucial, but if you end up with more noise in the image than you like, some noise reduction software may be needed, or you can decide to live with it. Most large prints (16x20 and up) are usually viewed at a distance that makes noise less prominent than the subject of the photo. Capture the image first, and worrying about post processing later. All the best post processing tools money can buy won't help if you don't get the image to begin with.
If you are taking a long lens (300 mm and longer) but haven't used one much, practice before you go. Get the hang of getting sharp images under simple and controlled circumstance before you try to get the shot of a lifetime in the field.