What are the requirements to get a secret clearance?

1. don't be a criminal
2. don't associate with criminals
3. Don't associate with foreign nationals

How long does it take? apparently forever... seriously though, the lag time on processing clearances is currently loooooong.

First and foremost you need to you have a need. Clearances aren't just handed out, there has to be a military requirement ( by MOS, by rank, or by duty position) for a clearance. Once you have established the need for a clearance, you fill out an SF86. This is done electronically these days. As of about 5 years ago, when I got out of the PERSEC game, it was done on a program called EQIP. Your S2 will get you started.
Once you start on the SF86, you need to be 100% honest, and answer the questions that are asked. If the question is HAVE YOU EVER smoked weed, and you tried it that one time in HS, you answer yes. If the question is IN THE LAST 7 YEARS, and that one time in HS was 8 years ago, you answer no. BUT NEVER LIE.

If you have a OK finances (no major unsecured debts, not behind on payments), a relatively clean record (no convictions, MAYBE an arrest or two), no serious mental health diagnosis (going to a counselor / therapist is fine. Even many actually diagnoses are fine - I recieved my last TS/SCI "pass" with an active chronic depression diagnosis - bit some diagnosis may cause you to be turned down. Most of these, however, will also trigger a MEB, so you probably do not have to worry) and you TELL THE TRUTH you should be OK.

#4 be responsible with your finances. Don't rack up large amounts of debt.

If you go to ClearanceJobs.Com they have lots of great factual information regarding clearances and everything associated with getting, maintaining, and losing a clearance (don't do those things!)

If you are an immigrant or family a recent immigration, be prepared to answer questions there. My last investigation I was married to a foreign national at the time and it was my longest interview to date. My wife is from a country with very positive and strong ties in the US and it still took a long time.

Application is on line. Also training about applying for clearance is available on line. Gather the documents you’ll need before you start the application. Fill in all the information with absolute honesty. If you come to a necessary fact you don’t have, save your application and go find the needed information. Come back and complete the forms. Pay careful attention to all instructions. Limit responses to exactly what’s asked for.

#5. DON'T LIE NOR PURPOSEFULLY OMIT information on your investigation paperwork.