Many musicians get into the habit of never keeping a pencil and paper around, but having something to write with is crucial in getting your ideas down on paper so you don't forget them. Then when it comes time to record your ideas, it's all there in front of you.
Many people start with the lyrics and then develop a melody from there. If you are new to songwriting, then this is a great way to start. Usually, working from the smallest element in the song to the largest, gets the job done.
For example, start with lyrics, then melody, then chord structure, then form, then arrangement. It's not easy to do the later steps without having laid the groundwork for it.
When it comes time to record your ideas, don't fuss over the details of your performance. Get it down quickly so you can keep the creative juices flowing. Then you can go back and refine your performance, make edits, ad instruments, and tweak the arrangement.
Before you get too far, it's a good idea to try the song out on a few friends and other musicians. They can give you feedback and ideas for making your song better.