Non-negotiable basics for soil mix and fertilizing in a first container garden

[Stella54]

I'm a complete beginner trying to start my first small vegetable garden in a raised bed on my sunny patio, and I'm already overwhelmed by conflicting gardening tips about soil composition, watering schedules, and companion planting. I've chosen to start with tomatoes, lettuce, and herbs, but I'm worried about making a fundamental mistake that will doom my plants from the start. For those with experience in container or small-space gardening, what are the absolute non-negotiable basics I should focus on first, and what common pitfalls should I avoid to give my plants the best chance? I'm particularly confused about how to properly prepare my soil mix and how often to fertilize organically.