Why is formic acid a stronger acid than acetic acid?
We are discussing the following equilibrium



We can make the acid a stronger acid by pushing the equilibrium to the right.  To push the equilibrium to the right we can 


destabilize the starting acid pictured on the left side of the
equation, and \ or
stabilize the carboxylate anion pictured on the right side of the
equation.


Comparing acetic acid ($\ce{R~ =~ CH3}$) to formic acid ($\ce{R~ =~ H}$), the methyl group is electron releasing compared to hydrogen.  Therefore the methyl group will stabilize the dipolar resonance form of the starting acid where there is a partial positive charge on the carbonyl carbon.  This should stabilize the starting acid. Further, this electron releasing ability of the methyl group will tend to destabilize the resultant carboxylate anion which already has a full unit of negative charge.  

Therefore, because the methyl group 1) stabilizes the starting acid and 2) destabilizes the carboxylate anion product, the methyl group will push the equilibrium to the left, compared to the case where the methyl group is replaced by a hydrogen.  Consequently, acetic acid is a weaker acid than formic acid.