1. HS oscillator is probably not set in your programmer
2. PORTB=Myvar will output your variable result in one shot using all 8 bits if your var is define as a BYTE sized one
3. and you'll need to modify your code if you want to display your results
4. Since we don't know your PIC model, you''ll probably don't need to set your PORTA as analog... More than often, it's already in analog when you boot the PIC.

Code:
' PicBasic Pro program to perform 
' 8-bit A/D conversion
'
' Connect analog input to (RA0)
' Connect LEDs to 8 pins of PORTB

DEFINE OSC 20 ' Sets clock speed to 20Mhz=> speed of your crystal not a/d

' Define ADCIN parameters
Define ADC_BITS 8 ' Set number of bits in result
Define ADC_CLOCK 3 ' Set clock source (3=rc)
Define ADC_SAMPLEUS 50 ' Set sampling time in uS

adval var byte ' Create adval to store result

TRISA = %11111111 ' Set PORTA to all input
TRISB = %00000000 ' Set PORTB to all output

loop: 
     ADCIN 0, adval ' Read channel 0 to adval
     ' possible that you need a pause here... few ms
     PORTB=Adval ' display result on portb
     PAUSE 200 ' wait 200 mSec
     Goto loop ' Do it forever
End