Save your Document Often
1. I would recommend to any beginner to SAVE after every step. I would even recommend saving after you spend a decent amount of time working on any given task.
Export As or Export Panel
2. Use the Export as, Save for Web, or Export Panel to Save PNG/JPEG. You can also adjust the file to fit in the artboard.
Adjusting File Size
3. If you need to adjust the size or output of the file don’t resize when exporting. Especially don’t resize UP. If you need a larger format scale the vector up or create a new artboard that meets the file requirements and scale the completed vector in the new document.
4. When you Export to PNG/JPEG just remember you are entering back into the world of pixels. If you are having issues with pixelation you can resize the artboard size to meet the print requirements.