I'm new to yugioh and I want to know what cards do I need to build a Kaiba deck?

Depends if you want to use JAP cards too or purely ENG. I rather you use some jap cards and make the deck look as complete as possible, especially with character themed decks.

Hmm.. Example Deck:

Monsters 24 (some are fusions and rituals too)

Chaos Emperer Dragon - Envoy of the End
BEWD
BEWD
BEWD
Lord of D
X-Head Cannon
Y-Dragon Head
Z-Metal Tank
Giant Germ
Diifferent Dimension Dragon
Kaiser Sea Horse
Kaibaman
Vorse Raider
Luster Dragon
Luster Dragon #2
Spirit Ryu
Mystic Tomato
Cyber Stein *if u get BEUD*

Fusion - XYZ, XY, XZ, YZ , Blue Eyes Ultimate Dragon *yes get it jap its worth it*
Ritual - Paladin of White Dragon

Spells 16-17 ish

Flute of Summoning Dragon
Enemy Controller
Shrink
Soul Release
Dimension Fusion
and etc etc

Traps 6 ish

Crush Card
DDV
Interdimensional matter Transp.
etc. etc