To begin with a positive note, screen tests are not something that is impossible to crack although it might seem tough and nerve wrecking for many. But remember you don't get these opportunities too often so you have to make the best use of it to demonstrate your talent to the talent agents, casting directors and directors. You should not be let down by the outcome of the screen test and always look ahead, you should always take it in a right sense after all the failures are stepping-stones to success. Keep in mind only one fact that you have to work hard towards your goal and having a positive attitude can do that. Apart from the attitude, one should be passionate and ambitious to achieve goals. Also enjoying the work you do makes the quest easier.
Do not forget to take your resume and a headshot most preferably 8"X10" with you to the audition. Some talent agents also need social security number, so take it with you. Sometimes the aspiring actors are called in the number of hundreds, be prepared to stand in line for hours. But most of the times you are called for screen test through an invitation. It is very common that you will get few pages of the script that you may be asked to read during the screen test. Read the script thoroughly and get into the character and ask yourself questions about the character. After getting into the skin of the character you should learn your lines properly using the best way you can.