With gig work, you're screwed if you don't plan for the lean months. Budget based on your worst-case income, cover rent and food first, then force yourself to save 10% of whatever's left and no excuses. Most people fail because they splurge on good months and don't be that idiot.
Costs are high; avoid them: no annual fee, no setup fees, no monthly fees. Prefer cards that graduate, report to all three bureaus, and have 0 dollar fraud liability and a 21-25 day grace period. Minimum redemption 1 dollar or less; foreign transaction fee 0 percent if possible.