You have to practice. You know how when you do math you learn the rules and then you practice them? Well riddles and word problems are like math. Each kind has its own special rules. They may not be written down. You may have to figure them out. You learn the rules and you practice. Libraries and bookstores have books with problems like you're talking about in them.
I just read some riddles and answers and I learn them. oh and reading helps a lot I used to be horrible at math and riddles and stuff but in 3rd grade I started to read a lot and I didn't read any thing like math books or anything just random books and all of the sudden im passing everything I went from ds to as. so try reading a little it doesn't have to be every day or anything. but it does help for some reason.