Problems with my legacy, need help please!

Hey i own a 1990 Subaru legacy gt. i recently bort if off someone knowing the clutch was a problem. but driving it home casued problems and the clutch is now not wokring at all. the car has now been left sitting outside for about a month and the battery has gone flat for some unknown reason, the alarm is playing up and sometimes wont lock or unlock the car. i charged the battery and tried to turn the car on, there was just a fast clicking sound coming from the engine. There was power in the car but it wouldnt turn on.
Could this be the starter engine or imobiliser? or is it something else.
any help would be appreciated
Thanks

Check to make sure that your battery isnt being drained by being grounded improperly with a hot wire touching metal first, Then try to recharge the battery with some one elses car. If it refuses to turn over after that, change the battery AND the alternater. With subarus i have noticed that once the battery goes bad the alternater will go as well. if you change both of those things and still having problems check the voltage regulator.

When you charged the battery did you clean the cables and terminals ?
Have the battery load tested, parts house or reputable shop.
If you get the vehicle running have the alternator tested also.
Check the connections on the starter to make sure that they are clean and tight.
Attach a test light to the red/yellow wire on the starter, turn the key to the start position and see if the test light illuminates.

You can remove the negative cable from the battery and connect a test light between the cable and the negative terminal on the battery. Make sure all doors are closed, if the test light illuminates then you have a short in a circuit somewhere draining the battery.

Hope this helps.