Well, I can help you with virtually anything conceivable regarding the bass, except for two things which I have never done: 1. slapping, 2. playing with a pick. EVERYTHING else within the world of basses, I can help you with, electric or upright (though my ability at the former is more substantial than the latter.) There’s no one way to play the bass, but I always play with my thumb resting on the E string. I never consciously did this but it became second nature over time and whenever I play anything on the E string my thumb temporarily lifts off of it, like a physical reflex. I have never played a bass with more than 4 strings (I mean, I’ve PLAYED them, but never owned one and never will, I really prefer 4 strings and see no reason to have more.)
The best advice I could give to anyone starting out on the bass is to PLAY ALONG with whatever music you like, as often as you can (if you want to get good.) DO NOT USE TABS, they are a waste of time and they encourage you to think in completely the wrong way about musicianship, tabs are useless, do NOT ever use them. Learn by ear. You need to develop the ear. You need to be able to follow along with chord changes and you’ll eventually learn intuitively that most pop and rock songs use fourths a lot, and other things about intervals, it’s all very confusing for someone starting out, DON’T try to absorb everything at once, it’s impossible. The ear is more important than knowledge of theory. Play along with a lot of Paul McCartney, Bruce Thomas (from The Attractions, Elvis Costello’s band), Leland Sklar, Phil Lesh, Carol Kaye, James Jamerson, Wilton Felder (Jackson 5 and Joni Mitchell’s Court and Spark), etc, I could go on and on and on, just play along and try to duplicate exactly what they are playing. Be patient. It will happen eventually, you just need to keep at it. Eventually you WILL get it. Also, something to keep in mind: if you continuously play along with recordings and directly copy other bassists’ styles, eventually YOU WILL develop your own stylistic touches intuitively and your own style will emerge. But you need the copying to form the basis of it.
If you have musician friends, jam regularly. Try to experiment with stuff, don’t just play simple repetitive songs, definitely improvise a lot. Don’t just play along with the chords when you jam - as the bassist, YOU control the chords as much as any other instrument. Outline chord arpeggios on the bass and YOU can be the chord changes.
Read the biography of Jaco Pastorius by Bill Milkowski and study his career closely, listening intently to all his playing, especially on the Joni Mitchell albums and the Shadows and Light live recording…listen very carefully to how he always ‘sings along’ on the bass with whatever else is going on, he plays so creatively and melodically but never drops the beat. That’s the goal you want to reach for. Jaco himself freely admitted that he developed his style through sheer chops-building and hard work playing along with recordings for years, not through some kind of innate God-given gift. He just played his bass all the time. He played along with whatever was playing on the radio in his room. He played along with the score to TV shows and the music on animated cartoons; he’d play along with advertising jingles; he just played along with everything. That’s all you gotta do.