I am relatively new to D&D. I've been playing a character in 3/3.5e and have been playing with others in 5e. The latter have basically been guiding me in how to build a character. I've read how Hit Dice are tied to a characters class, but I've been unable to find the answer on my own about where to find the a character's Hit Dice when building the character. I'm looking for more guidance on this matter.
You can see the relevant information in the Basic Rules and in the Player's Handbook, specifically:
Hit Points and Hit Dice (PHB page 12)
At 1st level, your character has 1 Hit Die, and the die type is determined by your class. You start with hit points equal to the highest roll of that die, as indicated in your class description. (You also add your Constitution modifier, which you’ll determine in step 3.) This is also your hit point maximum.
And, as you gain levels:
Class Features and Hit Dice (PHB page 15)
Each time you gain a level, you gain 1 additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total to your hit point maximum. Alternatively, you can use the fixed value shown in your class entry, which is the average result of the die roll (rounded up).
Each class describes what kind of die it grants you as a Hit Die. For instance, the barbarian class description (PHB page 47) states:
Hit Dice: 1d12 per barbarian level
As in 3rd edition, your Hit Dice are used to determine your maximum HP. Unlike 3/3.5e, in normal play outside of character creation or levelling up, your Hit Dice are also used as a resource to recover lost HP in a short rest (PHB page 186):
A character can spend one or more Hit Dice at the end of a short rest, up to the character's maximum number of Hit Dice, which is equal to the character's level. For each Hit Die spent in this way, the player rolls the die and adds the character's Constitution modifier to it. The character regains hit points equal to the total. The player can decide to spend an additional Hit Die after each roll.
"Spent" Hit Dice are recovered by completing a long rest (PHB page 186):
At the end of a long rest, a character regains all lost hit points. The character also regains spent Hit Dice, up to a number of dice equal to half of the character's total number of them (minimum of one die). For example, if a character has eight Hit Dice, he or she can regain four spent Hit Dice upon finishing a long rest.
Carcer's answer is quite thorough, so use that for details on Hit Dice (HD) mechanics. This is just in regards to your specific question.
You generally have as many HD as you have levels. If you multi-classed, the type of HD might be different between classes, (barbarians use 1d12/level and sorcerers 1d6/level,) but otherwise, it's fairly straightforward. for example, a Level 3 Barbarian and Level 2 Sorcerer multiclass would have 5 HD, 3d12 and 2d6. When you "spend" Hit Dice for healing at the end of a Short Rest, you select from your pool of dice how many and of which types of dice you want to use. You then roll them, healing yourself by that total. These spent dice are then unavailable until recovered during a Long Rest.
Using my earlier example multi-classed character: You could use 1d12 and 2d6 allowing you to heal between 3-24 by the end of the Short Rest, but you'd be unable to use those dice again until you Long Rest.
The following is the HD type each class receives.
- Barbarian - 1d12
- Bard - 1d8
- Cleric - 1d8
- Druid - 1d8
- Fighter - 1d10
- Monk - 1d8
- Paladin - 1d10
- Ranger - 1d10
- Rogue - 1d8
- Sorcerer - 1d6
- Warlock - 1d8
- Wizard - 1d6