A few notes:

1. Your monster's size can Tiny, Small, Medium, Large, Huge, or Gargantuan.  The hit dice depend on the size, from d4s for Tiny to d20's for Gargantuan.  All the other stuff is tweak able.

2. A monster's proficiency bonus is based on their challenge rating.  It starts at +2, and goes up every four Challenge Rating levels (E.G. At CR 5, the bonus is +3.  At nine it's +4, and so on).

3. In most cases for attacks, you take the ability score modifier the monster is using for the attack(If it's a spell, it'll use Intelligence, Wisdom, or Charisma.  If it's a weapon, it'll use Strength or Dexterity) and add the monster's proficiency bonus for the attack bonus.  For the damage, compare it to the damage of the weapons list and decide which dice work the best.  If your monster is using a weapon from the list and is larger than medium, consider adding an additional damage die for every size above medium it is.

4.  Misc stuff:  I put all of the different types of damage and conditions so that you can sort out what you want.  Immunity to a damage type means that damage of that type is reduced to 0.  Resistance halves the specified damage type.  Vulnerability doubles it.

In most cases(Except for Ogres), a minimum intelligence of 6 is needed to know a language.  In most cases, Perception is the most important skill proficiency for monsters.  The absolute average speed for monsters is 30.  Darkvision is really common for monsters and players.