My life is at a complete standstill ...

My job gets me by and will remain lucrative for another year or so. School starts in the fall. I'm in a (fairly) new city and my friends are busy people. My girlfriend no longer cares about me. I have no car so I stay at home a lot, but then again I wouldn't want a car because the economy is shit.

I'm a bright, ambitious guy but terribly lazy. I'm not a shut-in, like some in other threads, but I am definitely a member of the stoner-slacker class.

If something isn't happening in my life, I don't do anything. Surely, my life could be more interesting and fulfilling than it is now. So how do I make that happen?

Write something you want to accomplish on a sticky note, then keep it on your forehead until you accomplish it!!

When I have some spare time, I want to take acting classes.

Try to organise time for you to hang out with your friends, but I'm guessing you already tried that so make some new ones (in addition to your current friends). As school is starting at fall you should be able to make friends in your classes, but if school is too long to wait for join a club or something that interests you, like acting classes.