I'm going to play my first game of D&D soon. How do I get temporary hit points and how many of them do I start with?
You get temporary hit points from spells, feats, or features; you don't start with any
Every Player Character has hit points which they get at level 1 and represent their general well-being. How you get these and how many you get are explained in the section of the Player's Handbook that describe your class.
Temporary hit points are different from (really completely unrelated to) normal hit points though. The basic rules describe briefly the difference:
Some spells and special abilities confer temporary hit points to a creature. Temporary hit points aren't actual hit points; they are a buffer against damage, a pool of hit points that protect you from injury.
So how do you get temporary hit points? Well, spells, feats, or class features give them to you. You will know it because they will specifically say "temporary hit points" and tell you how many you get and for how long. Examples include the heroism spell, the Inspiring Leader feat, or the Storm Herald Barbarian's storm aura (tundra). For example, heroism says:
Until the spell ends, the creature [...] gains temporary hit points equal to your spellcasting ability modifier at the start of each of its turns.
If there are no active effects on you with this kind of wording, then you have no temporary hit points.
You do not get temporary hit points, unless an effect specifically tells you that you gain them. They are not a default thing that characters have. If an effect tells you to gain temporary hit points, it will also tell you how many you get.
Characters only gain regular hit points, which are explained (including how many you get) in your class section.