Don't use a metronome and don't practice scales! Separate art from academics because they do not relate.
The problem with people suggesting using metronomes is that they forget that academics is different than art and it requires another approach, an out of time approach. The objective here is to learn, not to perform, so never use a click when you are reading or learning new music. A click never allows you the time to learn.
Further, don't practice scales. They are THE bass cliche, the only element related to academic study that most teachers who do not know how to teach very well will give to their students. Scales actually don't represent tonality in specific ways. They are vague, they imply several different harmonic possibilities. They are bad practice methods.
Try chord tones, reading trombome music, and most importantly, find a teacher who can show you what to do. New players, or those with basic skills aren't qualified to teach themselves because they don't know their topic very well, that of music and what to work on. But, a great teacher will be responsible for what you should be working on, so make sure that your teacher is a really good one. He/she will be the one who doesn't ask you what you want to work on.
