Make sure you take samples from several spots in your garden and mix them together to obtain a representative reading. As with most crops, proper soil preparation is essential. If you have a large enough field, disc and cultivate to really work up the soil. If you have a large garden, do the rototiller thing. If you have a small plot, spade up the top 6 to 12 inches.
Garlic roots like to go deep, so well cultivated soil is a big help.
Mix in the organic matter and manure at this phase. After the deep tilling, we find a final pass with a cultivator that powders up the upper several inches of the soil aids in planting. Plant 4 to 6 weeks before significant ground freezing may occur. On the High Plains, we like to get going by mid-September, since snow by the end of September is not at all that rare here. Further east and south, late September and into October will generally do.