Figure out what's keeping your boss up at night, then put together a plan to fix it. If it's a small enough thing that you can do it without asking permission, just solve the problem and then tell them the next chance you get - this is ideal.
If it's a big enough problem that you need their approval to work on it, put together a pitch to present the plan to your boss. Structure it like an elevator pitch:
1. What's the problem?
2. Why is it significant?
3. How will you solve it?
4. How long/how many resources will it take to solve?
If you take the initiative to identify problems and solve them yourself, you'll rise above 90% of the people you work with immediately, PLUS you'll gain more control over the projects you're working on.
But remember, it's not enough to identify problems. If you don't come prepared with solutions, you won't be seen as proactive - just annoying.
The more of an entrepreneur you can be at work, the more successful you'll be.