Some questions to consider which will narrow down the available options:
- Do you want to design and/or build circuitry, or just write code for an existing board?
- Do you want to have lots of peripherals on the board (e.g. blinkenlights, buttons, display, ethernet interface, analog/digital converters, ...) or would you rather add on what you need?
- Do you want to make purely data-processing things, or do you want to control something in the real world (like a robot)?
- Do you want to do any audio or video processing?
- Do you want an embedded processor (runs software), or do you want an FPGA (acts like reconfigurable logic hardware)?
- If you are going to be writing software, do you want to do it in assembly? C? Something higher-level than that?
- Do you plan on doing something complicated that you think will require a fast processor and/or lots of memory?
- Do you require an open-source compiler/development environment or are you OK with using the board/chip vendor's?
- Are you at all interested in working with analog electronics or do you plan to stick to digital?