sing along to scales on the piano, or any other reference instrument. This will help calibrate your ear, but it doesn’t seem like that’s as much your issue as poor technique.
GET THEE TO A VOICE TEACHER.
If that’s not possible, there are other solutions that will work but are no substitute for live feedback from a professional.
Grab a voice exercises CD (I recommend Vol 1 of this)
and practice with it. Most pitch issues are either a) poor ear b) weak breath control c) improper technique going into a pitch
sing from the diaphragm, with proper support…improper support will cause a tendency to go flat.
When going to a pitch, consciously attempt to come into the pitch from above, even if truly coming from a lower note. So AIM HIGH and settle down into the pitch. You might end up a touch sharp at first, but it will self-correct. And slightly sharp always sounds better than slightly flat. Don’t know why, but it is so.
There are some real voice teachers here that could clarify or correct my comments, but I believe they’re a good starting point (I am a music teacher and arranger, have run lots of choruses et al, but i am NOT a voice teacher per se.)