There are several "Which FFP's should I join" threads. Starting questions are:--

Best Frequent Flyer Program for you?
The answer to this question depends very much on your personal travel profile and objectives.
- what is your definition of best?
- do you value miles/points (or equivalent) for redemption over gaining status?
- do you fly enough to gain any meaningful status?
- what do you want to use the miles/points for (award, upgrade, hotels, what ever)?
- what sort of flight award is of interest (destination, class of travel)?
- are you interested in topping up by paying cash for miles?
- which airline and where do you normally fly?
- are you primarily a leisure or business traveller?
- where do you live?
- do you have other ways of earning points in the various programs, e.g. credit cards, shopping, etc,?
- etc, etc, etc
(above copied in part from a post by dunk)

There is a big difference between a frequent flyer and a frequent spender getting freq flyer miles from a credit card. For questions about which is the best credit card to sign up for, to get frequent flyer miles, please do not post here, but refer to other threads on this topic

Questions
For members asking for information, to help people to assist you, can you please provide:
(1) What is most important to you in a FFP?
(upgrades on travel, priority services when flying the airline, extra baggage allowance, good award redemption rates, better award access, free - discounted lounge access, etc.)
Reply:
(2) How many miles do you usually fly each year & in what class? How many flights/sectors?
(<25000, 25000-50000, >50000 miles - <25, 25-50, >50 flights?)
Reply:
(3) What types of fares do you usually buy ?
(First, Business, Premium economy, Economy, cheapest)
Reply:
(4) Can you choose your airlines and/or class of service? Do you travel for work and/or pleasure?
Reply:
(5) Which routes do you fly most often
(US Domestic, Transpacific, Kangaroo, in Asia etc)
Reply:
(6) What is your home airport?
(SFO, SCL, London LHR, HKG, Singapore SIN etc.)
Reply:
(7) Do you have FFP status of any kind in an airline? What is it? Do you have any miles banked in a FFP?
(AA Executive Platinum, UA 1K, LAN Comodoro, etc)
Reply:
(8) Preferred Airlines
Reply:

Similar sticky threads
If you are sure a Star Alliance or OneWorld Alliance or Skyteam airline FFP is for you, please post in those forums, as more Star/OW/Sky experienced people are likely to answer your question.
Star Alliance forum http://www.flyertalk.com/forum/star-...post-here.html
OneWorld Alliance forum http://www.flyertalk.com/forum/onewo...help-here.html
Skyteam Alliance forum http://www.flyertalk.com/forum/skyte...i-sign-up.html
Information Desk forum http://www.flyertalk.com/forum/infor...help-here.html

Major airline alliances are
www.staralliance.com web site and the Flyertalk forum Star Alliance
www.oneworld.com web site and the Flyertalk forum OneWorld Alliance
www.skyteam.com web site and the Flyertalk forum Skyteam