There are a couple things you could do to dramatically improve yourself. The first would be to sing with confidence. Let your voice explode out of you… When you stifle yourself, you can rarely hit a tone. I had this problem when I was first starting.

The second thing is to record yourself and listen. You can easily tell what you're doing wrong when you hear yourself. It may be tough at first as it will probably sound terrible, but it's a great way to learn. The only way I got good at singing, was to hear myself over and over and focus directly on the things I could hear were wrong.

If you're actually tone deaf, there's not much hope in that department, but you can often fix this just by listening a bunch.

Well, if you don't have a talent for it you probably won't be fantastic or anything, but you can get better. Try to practice simple warm-ups every day. Sing the solfege a few times for basics. Never try to sing too high or too low, you'll damage your voice. Sing scales, get an idea of your range. Then you can decide what to sing. Practice songs in your range, it'll make you better. If you sing scales every day, you can probably increase your range. There are apps that can help you with the solfege.
Good luck! And don't listen to the people that say you can't do it, you can!

Sing with power from your diaphram. A common misconception that cartoons and that crap give people today is that you breathe by puffing your chest up, and that when you breathe in, your stomach should contract. In fact, breathing is like filling a glass with water; the water shouldn't start at the top and work its way down, rather it should start at the bottom and only fill your entire chest when you're full of hot air (he he). Also, if you think about it, when you breathe in, your stomach should go OUT because it is filling with air. I have no idea where the misconception came from that you suck in your gut to inhale. When you're breathing naturally, you can see your stomach go in and out the right way.
As for vocal production; you have to stand or sit 'as if someone had run a flagpole down your spine'. This unholy description given to me by a musical director actually helps. Another one; sing as if there was an oreo standing up inside your mouth. Basically, this means that you should always have your mouth open very wide. We had a guest singer come once who never opened his mouth more than an inch; he sounded like a moron.
When you're doing low notes, you have to feel the tone coming from your vocal chords, in the middle of your throat. It's important not to sing to loud from this area, or do something like scream, because it's very easy to bruise or tear the muscle. In you comfort zone of notes, the sound should come from directly behind the point that your teeth converge when you bite down, essentially in the middle of your head. High notes come from the nasal area, which is the secret to how the ultra-sopranos make that classic opera sound that shatters glass.
My piano teacher's daughter is a graduate in vocals from Boston University. I've never talked to her much, but she's often wandering about doing scales while buzzing her lips. I've tried this a few times (when I was alone) and I have to say that she must be really good, because it's hard to do without doing it at the top of your lungs. A fun thing we did once was start 'aaah'ing as low as you can, then start rising (try to make no break in the sound when you move from your 'chest voice' to your 'head voice'). Go up untill you're practically screaming and then start to come down again. Also good are scales, arpeggios, and individual intervals of a whole step, a half step, a fifth, tritones, minor thirds and major sevenths.