I guess that you have not had much answer on the FFP issue, as we are used to someone with limited knowledge of FFPs but not with what would seem to be virtually no knowledge. But I'll start on this.

First, it is not a good idea to join all the FFPs that you can. It is better, as a general principle, to identify within OW the FFP which is going to be more useful to you and put all your OW miles in it.
FFP miles will enable you to obtain free tickets but also to enjoy some perks, such as, for instance, lounge access if you do enough miles in a year to reach a particular elite status within the programme. Given that you are doing a RTW, you should make enough miles there to reach elite status.
In your particular situation, it would seem to me that AA would have the most useful programme, so you should read up on it on the aa site, hang around on the AA board and also look as Fewmiles' Unofficial Guide to American Airlines AAdvantage.
Once you understand the basic mechanics of AAdvantage, you may want to read up threads on the AA board on the Platinum challenge, which is an accelerated way for you to reach elite status. Given that you are doing a RTW in economy, getting elite status via a Gold and/or Platinum challenge will be especially useful for you, as you will earn more miles from your trip, as well as have access to lounges on international flights once you reach Platinum. This should be relatively easy to take a Plat challenge if you are doing a RTW, but there are some pitfalls to be aware of (best time to start challenge, which flights are eligible for how points on this or that airline, etc...).
All of this can seem a little daunting at first but you will probably find it very rewarding in the end.