That's kind of a tough question, because it varies greatly based on where you go and on your job focus. The first part is completing boot camp and MCT, which is hard but not impossible. Then, if you're a 267x you'll go to DLI for your language which is very easy (physically) but can be mentally exhausting, depending on you and your language and everything else. Monterey, CA is beautiful. If you pass your language and the DLPT (Defense Language Proficiency Test) then you'll proceed to Goodfellow AFB (read: hell) for some follow-on training. If you are a 2621/2651 you will skip DLI and go straight to hell/Goodfellow. This is a recent change, because previously 21s/51s went to Correy Station in FL, but now they don't - you might see older information saying that they go there. Following completion of your training in Goodfellow, you'll go to either a Radio Battalion (1st - Pendleton, 2nd - Lejeune, 3rd - Hawaii) or a Letter company (part of MCSB). There are ones and twos that go other places, but I don't know much about that and it primarily applies to 2621s, I think. I also don't know much about Radio Battalion, except that it probably isn't as bad as people say it is. I know that it is very Marine Corps-y, meaning unit PT and field time are emphasized more than at a letter company, whereas if you go to a letter company, your day-to-day can be very exciting one week and then super dull the following week, but it is consistently a "day job" in that most of the bn wants you to use your brain and you work in an air-conditioned building. I can't get too much into the job itself, but I've run the full gamut from 'love the job/hate the Corps' to 'love the Corps/hate the job' in my several years at a letter company and it really just depends on what's happening.

You will normally know about clearance issues by the time you get to DLI or Goodfellow. It should be fully processed before you hit your duty station.