If you plan on singing the national anthem, it would be a good idea to study, learn and know the lyrics and their meaning. Being able to deliver a song with the proper emotion requires knowing the meaning of what you are singing about. Read the lyrics until you know them well. Writing the lyrics down can help you remember them.
Finding the right key to sing in can take some experimenting. Start with the hardest note to hit and work from there. If you can hit that note comfortably, that will help you determine a good key for you. If you are singing in a key that is too high or too low, it will affect your ability to hit notes that are in a broad range.
The more you practice singing the national anthem, the more you prepare your voice to hit the proper notes. Your voice is like a muscle in that the more you use it, the stronger it gets. If you are planning on performing the song, practice it every chance you get so that the lyrics and hitting notes on key become second nature.
If you are singing the national anthem in a big arena, use earplugs. Monitors are often not close to the singer in large arenas, making it very difficult to hear yourself. There is typically a one- or two-second delay in stadium sound systems, so you have to concentrate on what you're singing, not what you're hearing; otherwise, it can throw you off.