and primer3 works well not forgetting that if you are sequencing the gene then set your primers at least 50 bases outside of the exons as the primer and first 30 bases will sequence badly
In additions to the links that were given to you above, I would recommend to use a soft like SnapGene if you are using microorganisms. It helps a lot to design primer easily on a known sequence.
On my own, I use usually 20 to 30 base-primers so that the annealing T°C is about 50°C, because I usually use the PrimeStar Polymerase at this T°C. In other words, its best to adjust the T°C of your primer to your pcr method (and there are many :).