Welcome to FT!

You have several questions here.

1. To get credit for flights, you have to join the airline's frequent flyer program. You do not need any kind of credit card to do this.

2. Generally, you have to be a member when you fly. Airlines may give credit for recent flights as an incentive to join if one learns of a program while on a trip, but nowhere near six months back. Airline policies vary. Check the airline(s) in question, via their Web sites (the most authoritative source) or here. On FT, most active airline forums have "sticky" threads at the top to answer common questions. Read those first. Then scan the other threads in that forum, using the FT Search feature (it's not perfect), to try to find an existing answer before posting a question.

3. Once you are a member of a program, you can often get a credit card to add to your mileage total. You do not need one for flight credit. Depending on your spending patterns, travel patterns and desired awards, you may have better credit card choices. (In some cases you can apply for a credit card and program membership in one step - but, again, you do not need a credit card to join a frequent flyer program.) There are FT forums for credit cards, as well as discussions of each airline's credit card options in the airline forums.