Don't listen to the guy that say 2-3 days. Cycling your tank will take anywhere from two weeks to a month.
Fill your tank with your mixed saltwater (specific gravity around 1.024), your sand/rock, and drop a whole, uncooked (dead of course) cocktale shrimp in there. The shrimp needs to rot in there.
Next you need to buy a test kit that tests for ammonia, nitrite, nitrate, ph. After about a week or so your ammonia will spike considerable (very high reading on test kit). Wait until this reach zero (another week or two). This is when your nitrites/nitrates will spike.
When the nitrites get to zero, your cycle is pretty much done. You will now do a large (20-30%) water change to lower your nitrates.
You are now ready to add your cleanup crew of snails and cleaner hermit crabs
ps- also make sure you test your water at leasty weekly. You will eventually need the following test kits as well: phosphates, alkalinity, calcium.