Try songs out at a variety of pitches as you're learning them; different songs can suit different parts of your voice - and different also on different days, of course.

Even fairly experienced singers often still have weaker notes at the 'join' of 2 different 'voices'. Mine is the G below middle C; so I always try to avoid pitching songs such that there are long held notes on that G.

Another good tip is, to reach notes at the higher end of your range, hold your head slightly down (in a natural, relaxed position). It's tempting to look up towards the ceiling/sky when trying to reach high notes, but that constricts the breathing mechanism and actually makes singing them more difficult.