Monster Cards
There are many types of monster cards, some of which are effect, ritual, fusion, and normal.
Here is a diagram that shows the layout of a monster card.
For all monsters cards you must look at the number of stars on the card to determine how many monsters you must sacrifice to bring it out. If the monster has 1 to 4 stars you don't have to sacrifice any monsters to bring it out unless the card says otherwise.  If the card has 5 or 6 stars you must sacrifice one monster to bring it out.  If the monster has 7 or more stars you must sacrifice 2 monsters to bring it out.
For normal type monsters the only thing you need to worry about is it's level to bring it out to the field.
For effect monsters you must read the text on the card to see what it does.  Some effect monsters have flip effects which only activate when the card is attacked in face-down defense position.  You must also sacrifice the required amount of monsters to bring these out to the field.
For fusion monsters you must have polymerization and the required monsters to bring the monster out.  Some fusion monsters also have effects, but most of them don't.
For Ritual monsters you must have the ritual card and the right number of monsters whos stars equal the same amount of the ritual monster. Some ritual monsters also have effects.
There are also five types of attributes.  They are dark, light, earth, wind, fire, and water.