Knowing your guitars parts well before you begin singing can make it much easier to sing and play simultaneously. Not having to concentrate on your chord changes or picking so much will allow you to focus more of your attention on giving your best vocal performance. Remember, playing guitar, especially when trying to focus on another task like singing, requires that you use your muscle memory.
Before you even begin to play and sing, imagine what the song is going to sound like. It does not matter if it is an original song or a cover of a popular song. Imagine yourself singing it and playing, and think about what you want it to sound like. Doing this will help you a great deal if you just take a few minutes to do it before you even pick up your guitar.
Write out the lyrics to the song in a way that is easy for you to read. If you do not read music, you can simply write the song out like a poem. Leave spaces between each line so that you can add chord or note changes above the words. Separate the verses and the choruses and label them accordingly. Once you have done this, you can practice by singing and playing while looking at your music. Since you don't have to memorize everything right away, a chord and lyrics sheet will make it easier to focus on what you need to do and allow you to work on more difficult sections independently.
One mistake that many musicians make is not taking practice seriously enough. If you intend to perform your music for an audience, you need to practice as if you are going to perform, even if you practice in your bedroom. If you are planning to stand up while you perform, do not sit down to practice; it will make your real performance in front of an audience feel awkward and new for you. The way you breathe and the way you play has a lot to do with your position, so trying to alter that the day of a show will probably not produce good results.