having bad credit and having a short credit history are two different things, the most important part of this game is to pay your bill in full on time EVERY TIME.
collecting miles is not a sprint, you may ready some stories here of peopleing getting a bazillion miles in 3 months but the key is consistancy, first step is make sure all bills are paid and on time and clean up any "old messes" then establish a good payment history, strat with fee free cards that may not offer rewards, in time add some rewards cards 1 or 2 at a time, enjoy the rewards.
remember the reason credit card companies offer these great rewards is to intice new people to sign up, you have to think about what works for you, some of us here game the credit cards and get stuff while paying very little in annual fees and nothing in interest or late payments, others gt excited about the free stuff but dont manage their situation and end up in debt with nothing to show for it.
take your time, establish your credit, get rewards,
Some one new wanting to get 6 cards is like someone wanting to get in shape on new years day so they say i should run 50 miles today, how about start with 1 mile and come back tomarrow and run one more