assuming you mean starting from no clearance? apply to a job that requires one. the catch-22 you'll find yourself in will be that a lot of positions will require a clearance of some kind before you start (especially in a top secret environment), so you'll be sitting around anywhere from 3-9 months (maybe longer) waiting for the background check process to complete before you're actually 'hirable'.
there's a good reddit (securityclearance, I think) that I would recommend going through to get a sense of how it works.
there's a good reddit (securityclearance, I think) that I would recommend going through to get a sense of how it works.