What word should I use to tell someone to look straight when they are looking down while driving or while walking around? Would "look up" be okay? What would it be if they are looking right or left?
The standard expression is:
"Look where you're going!" (where watch and mind are commonly used instead of look)
...which can be applied to someone on foot, cycling, or driving, for example (it's usually preceded by "Why don't you" if they've just bumped in to you or your car).
For a single-word preposition, "Look out!" is normally used when the speaker is aware of some upcoming danger that he thinks his audience hasn't seen.
It's not very common to say "Look up!" (or down, etc.) to indicate the direction of danger. You normally only see that on traffic signs for pedestrians warning them to "Look Left" (or Right) where they're about to step onto the road (the sign indicates which side the prevailing traffic will be approaching from).
Look ahead is the most generic, common phrase I could think of. The real answer is that it depends - or as J.R. commented, it's context dependent (it is English, after all).
- If the person is looking down, then tell them to look up. (To allay some possible confusion, this (to an English speaking native) would not be a directive to look at the sky.)
- If they are staring at the sky, tell them to look down. (As with look up, in this context look down would not mean look at the ground)
- If they are looking to the right or left, tell them to look ahead.
- If they're staring in the rearview mirror or have twisted around to see what's over their shoulder, tell them to look forward(s).
There are also plenty of expressions that don't use the word 'look'. You could say 'eyes forward', 'watch where you're going', 'watch what you're doing', 'stop shoegazing' (when walking), 'watch the road/traffic (when driving).