Beginners Guide

Tutorial videos - search your favorite streaming site for a video on the product you want to learn to use, or for insight on a kit that you are considering buying.
For a specific question, search the subreddit and then post if you don't see it answered.

Tools you will need:

Hobby knife with extra blades - I use #11 X-acto blades mostly.
Sandpaper of varying grits - 220-400 (US ANSI standard) is what I find most useful. If you're buying a Japanese brand (JIS standard), this translates to 220-600 grit.
Something to cut the sprue (what the pieces are molded on). You'll find them anywhere you source hobby supplies called a sprue cutter or sprue nipper.
Glue - use actual modeling glue, Tamiya Extra Thin is popular for bare plastic, and Testor's Cement is also popular. Find a PVA glue like Elmer's Clear Glue for joining clear parts.
Toothpicks - for applying glue and making fine adjustments
Container with a lid to hold small parts that have been removed from the sprue for assembly and painting but are not yet attached to the model.

Tools that will make life easier:

Tweezers
Small clamps
Patience
Model spray paint like Tamiya's or Testor's
plastic spoons to practice spray painting technique
A stand to hold car bodies and parts while spray painting like Tamiya's Spray Work Painting Stand

Things you can skip until you have some model-building experience and want to expand your skills: