(This will get better over time. I'm sick of typing this out. Sorry it's so bad.)
1. Go to google, search for your image, and save your favorite one.
2. Open that image in your favorite image editor.
3. Make sure color is not in indexed (switching to RGB is a simple solution if it's a gif. If it's a jpg, no worries). You can switch to greyscale if you have to.
4. If you want a high-detail stencil, skip this step, or play around with the setting on the blur. Apply a Gaussian blur to the image. If you're not satisfied after step 5, ctrl+z until your image is back to unblurred and play with the blur settings.
5. Find "Threshold" wherever color editing options like Brightness/Contrast, Saturation, etc. are, and play with until you get the b&w image you want.
6. VERY IMPORTANT INFORMATION IN THIS STEP: Now bridge your islands. This means make sure no white is completely surrounded by black. I always do it while cutting out, as opposed to on the computer, so if you're like that, don't do it now. If you want a hella fancy stencil and understand simple words like "spray adhesive," you don't have to worry about islands at all.
7. If you want the image to be huge, google programs like "PosteRazor" or "Rasterbator." Now print and cut.