Hello,
Welcome to the fun world of the upright bass. Some techniques to play the upright are different than the electric. Most people play a four finger technique on the electric, but on the upright, most people use a technique of playing notes in the left hand with the first, second, and fourth fingers. It is too big of a stretch to play a half step between the second and third. Hopefully the bass is well set up with a good bridge and has reasonable playability. Proper left hand technique and a good stance are essential for playing in tune. It is important to start with good technique so you don't learn bad habits that will be hard to change later. Since you stated that you cannot afford a teacher, I will suggest some videos online.
Here is a Youtube from John Clayton about the left hand position:
John Clayton, Finger position on a bass - YouTube
Paul Sharpe has a wonderful series of videos titled "First Year Technique....Double Bass" with a variety of topics including stance, left hand, endpins, bows, etc. They are on the D'Addario String company website.
theLessonRoom.com : Resource Search
Good Luck