Welcome @jennifer aola 

To answer your first question, which is pretty straight forward, is the range of the violin extends from G, the lowest open string, upward nearly four octaves.  This is assuming standard practice in the western hemisphere where typically tuning to equal temperament using the standard frequency of A₄ = 440Hz.  There are different kinds of tunings but these are much less common.

Your second question is a little more complicated.

If you are just talking about a comparison to the keyboard intervals to violin (and the other orchestral string instruments), learning the intervals is more or less included with your basic scale work.  However, there is quite a lot of theory which can be helpful which you can delve into concepts like the circle of fifths, keys and key signatures, and how music is constructed.  These are not, by the way, concepts that are exclusive to strings.  This is an area of music called music theory which helps to understand all music and how your particular instrument works within that structure.  There are entire music courses which teach theory.

But to help you with how intervals are applied to strings, the idea is really about steps.  I can refer to the piano, because you seem to be familiar with the piano.  The interval steps are the keys on the keyboard.  So for example, playing the natural, or white keys, are whole steps.  When you go from a white key to a black key, which is normally a flat or a sharp, you are playing a half step.  

String instruments also have whole and half steps.  But since there is no keys or frets to use on the orchestral strings (violin, viola, cello, bass), we talk about finger spaces.  Generally, on a single string, whole steps are played by extending a full space from one finger position to another on the string.  The half steps are when you play notes that are together, or touching.  

This is a fairly simplified explanation, but it is helpful to know that all instruments use intervals.  It is part of music structure and not necessarily instrument specific.

I hope this helps.  Please ask any questions.  We are happy to help!