Objects in motion tend to remain in motion at a constant velocity unless acted upon by an outside force; objects at rest tend to remain at rest unless acted upon by an outside force.

Sir Arthur Eddington observed the apparent lack of content in Newton's first law by saying, facetiously I think, that "every particle continues in its state of rest or in uniform motion in a straight line except insofar as it doesn't"

Newton's first law of motion seems redundant in view of the second law: since indeed F=ma, if no force is acting (and the object has mass), then its acceleration must be , and it must keep its current velocity.

So why is it there?

Newton's laws of motion aren't a description of a natural phenomenom. Forces aren't observable physical objects, except by application of Newton's laws. Instead, the 3 laws of motion can be considered an axiomatic definition of what force is.

The first law just tells you how to observe a force: just look for any object with a non-constant velocity. Now that you know what a force is, you can proceed to measure it using the second law.

Newton was well-versed in the axiomatic method; his 3 laws are a nice example of how it can be used.

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Either that, or Newton thought he'd look silly with second and third laws, but no first law to back them up.