1). Good question. But let me ask. What kind of spells are easier for you?
2). You don't necessarily need materials. But if you decide to use some you need to know the purpose that each possible material is for. Pick materials good for what you are trying to do. For example: ritual - use an athame; cleansing; white. Candle; banishing; black candle.
3). Personally I think you should always have full focus.
4). If it doesn't work the first time you try it do not get discouraged. Not all spells are going to work the first time you try them, whether it's one you made or one you found.
5). Technically yes you can make a spell about any thing. But that doesn't mean it'll work. I can make a spell that makes pigs sprout wings and fly. That doesn't mean it'll work and the pig will start flying. I can make a spell that makes me a god. I'll be a god in my own little world in my mind, but that doesn't make me an actual god. Be mindful of what you are trying to make a spell for. Try to keep it logical, but don't let that logic hold you back either. It could even be a spell that insinuated one thing by looking at it written or hearing it, but when done properly you find out what it truly does. Like the broomstick spell to fly. It implies that you can get on a broomstick and fly away. Not going to happen in this realm. The true broomstick spell to fly is meant to help aid in astral travel. I hope you get what I'm trying to say about what the spell can be made for.