Here are some of the basic things that you should have on hand:
construction paper - sissors - pencil - envalope - glue
Pick out all the colors and patterns that you want for your card. Try to pick colors that match with the patterned paper that your useing, same goes if your using multipel patterend paper.
Cut out your main piece. If your planning on putting your card in a envalope, measure it slightly smaller, so it fits nicely and not to snug. Than cut all the other collored and patterned papers that you picked, make sure you measure it with your main piece before cutting! Cut nice straight lines cause when you glue it on, you'll really be able to tell when its uneven!
When you're writting *Happy Birthday, or anything eles that you want to write on the front of the card, first write it out litghtly with a pecil, so you can just trace over it with your pen.
Set up how you want it to look before gluing any thing on. Then glue!
I like to add string to some of my cards, it really makes a great look!
You can make extravagant cards or simple ones, it will always put a smile on some ones face! :)