Changed the alternator ,now the car wont start

Assuming that your battery is good and it can crank the starter motor, Check the ignition system of your car.(Spark plugs - see if spark well'; Ignition/spark plug cables - for insulation breaks; distributor cap/rotor; and finally the ignition coil). Also check the carburetor for the right air-fuel mixture)
By the way, check your spark plug cable sequence.. It must be placed 1-2-4-3 or 1-3-4-2 according to
to the cylinder/pistons starting from the front.