Warm up your voice. It will be difficult to jump "voice first" into any song without warming up your vocal instrument. Just as physical instruments such as guitars or violins need to be tuned before they can be played, the same goes with your voice. Choose a couple of easy warm up songs that are both short and stretch your vocal range to properly warm up.
Gauge your vocal range. This can be done simply by being honest with your vocal range when performing warm-up scales. If the highest notes are comfortable to you without straining your voice, then you are probably a soprano; if some high notes are comfortable but not the highest, then your vocal range may be alto. Tenors are generally medium lower ranged notes while the lowest notes belong to bass-ranged voices.
Choose songs to sing from artists within your vocal range. You may also be able to change the octave (range) of the music to make the song more comfortable for your voice.
Practice every day consistently. Everyone knows the saying that "practice makes perfect." This saying is especially true when someone is aspiring to sing well. Sing along with your favorite songs and artist while the music is turned down so that you can hear yourself. If your voice is not blending with the music or is the wrong pitch, stop the music and find the pitch with your voice until they match. Critiquing yourself and making adjustments when you make mistakes will only improve your voice in the long run.
Seek outside criticism. After you have practiced on your own for a while and are confident you can sing a song in front of someone, take your act "on the road" by allowing others to hear your voice. Before you begin singing, ask your audience for constructive criticism at the end of your song. This is another good way to improve as your singing voice will likely be for an audience of different people.
Hire a voice coach. Vocal coaches will help you to control your voice better and also give you professional input. They also will be able to help you to stretch your vocal range as well as give you some helpful tips on how to take care of your voice.