Hi nida.sahar,
Most Business Schools have several application "Rounds", so if you miss an early deadline, you can still apply for a later one.
Each School lists its application deadlines on its respective website, so you can find the specifics for any particular School/Program without too much trouble. For most US Business Schools, the Round 1 deadline is in early October, the Round 2 deadline is in December/January, IF there's a Round 3 deadline...it's usually in February/March and some Schools even have a 4th Round...in April/May.
Since most Test Takers need 3 months (or more) to study for the GMAT, beginning your studies NOW would give you plenty of time before the typical Round 1 deadlines.
You might want to do some research into the particular programs that interest you, so that you can plan out a longer timeline (which would include studying for the GMAT, taking the GMAT and working on your applications).
Testing Centers tend to start "filling up" their appointments during late summer through the end of year, so you sometimes have to plan ahead (sometimes months ahead) to book a specific day and time for your Test.
GMAT assassins aren't born, they're made,
Rich