Find golden gates around Gotham to buy characters - but you need enough gold bricks to build the gates. Some characters unlock during story mode also.
Scan your map for the gates.
Re hints - I assume you mean red bricks - these are also situated around Gotham. You need either a special character to unlock them (Joker/Riddler) or you need to do some kind of activity. The first one I found was the x4 stud multiplyer which is by the police station
Scan your map for red dots - these are the red bricks
The chances of anything coming from mars are a million to one but still they come! GT: Lizziekins http://blueblood.enjin.com
Some of them, yes. The real issue you will run into is that it's hard as heck to explore properly until you beat the main story. I would recommend this: beat the story, don't worry about collecting besides some studs.. Afterwards, use superman to fly to each beacon location to unlock the full Gotham city map. Then, fight joker and riddler (former is at ace chemicals, later is on Wayne tower top), use studs to buy them afterwards. Using those two, go to each red brick location, at least the ones for the multipliers, minikits finder, peril finder, red brick finder, gold brick finder, and maybe stud magnet. Go find all the perils in Gotham plus a few gold bricks. Unlock aquaman, man-bat, lex luthor, penguin, and poison ivy (I think they will all work fine). Go replay the levels to get all the minikits, perils, and super hero status. Finally finish collecting in Gotham itself.
That is generally the way I went about getting 100% in the game.
The harder the game, the more likely you will scream and try to throw the game out of the window.