It looks like you're new here. If you want to get involved, click one of these buttons!
Can I get better or it's just I am not good at singing?
Some tuners can be used on the voice. You have to have a mic input for the tuner, and that may be how you use it on your guitar. Your webcam mic may work for this.
Another option is a program like Sing & See. That will tell you whether you are too low or too high. Programs like that can be a little discouraging, because you will almost always be a little low or a little high, but if you just use it as a tool to help you learn, it could be helpful.
You're not using enough diaphragmatic support. You will feel a little bit of downward push on your insides all of the time when you are supporting, and you will need to push down a little more, or a lot more as you sing higher. When we are pushing down, we have more control over the air we are letting out. We want that air to be compressed just a little bit, and under very fine control. That will help to prevent losing your breath, because you will be letting the air out very slowly and controlling it at all times. Your breath control will make it easier for you to sing longer notes in better tune.
The best suggestion for being able to tell whether your pitch is right is to first tune your guitar, and then play random notes on the guitar and try to match them with your voice. If your tuner will pick up the sound of your voice, then that would be helpful. Watch the tuner and adjust your voice until it is on the same note as the guitar and neither flat nor sharp, but in-tune. Repeat again and again and again as much as you can. Record yourself as you do this, and listen to the playback so that you can learn to hear what you are doing that is right and what you are doing that is wrong. Learn what you are having to do to get in-tune with the guitar and the tuner.
Teach yourself to build this skill. It is a skill that is learnable, but you must use all the patience that it takes to improve your listening skills and your matching skills and your singing skills to put this all together. You CAN do it, but you need to supply the time, the effort, and the patience. There is no substitute for good, old-fashioned practice. It will help you to improve.
As you continue to practice and use these tools to help you learn, you will eventually learn to hear it with your own ears and no longer need the tuner or to continue this type of practice. First you have to learn to tell the difference.
All the Best!