Clothing is important, Wearing over bright colors can distract the viewers, Wear neutral colors or simply all black or all white is the best option.
You need to keep your phone steady for the headshot, get a tripod or a friend to hold the phone for you.
Stand closer to the window so natural light can lit your face properly, donot stand under direct sunlight to avoid weired shadows.
Practice before clicking in the mirror, decide whether you want to smile or not but stay in your real personality for best results.
We all are not a model/professionals, So stop worrying and Try with different poses, backgrounds or lighting to find your desired results.
If you don't have a nice background use apps like Remove.bg or canva to remove your image background and replace with something you like.
Our phone lens is not that clean and a greasy lens can ruin your photos, take a cotton cloth and clean the lens carefully.
Use any editor like snapseed or lightroom to tweak some settings like exposure, highlights and shadows.
When you are satisfied with your best outputs save it in a different folder so you can use it whenever required.