Just about anything can be a pendulum. Some people suggest a metallic chain for conducting energy, but others say string or leather or whatever that allows the weight at the end to swing freely works great. The weight, again, is whatever you like. I purchased my first one as one that would simply look better than a key on a string, for example.
I went into experimentation with an idea of what would be "yes" or "no," but some people suggest asking the pendulum. The general 'yes' and 'no' are either a swing direction (back and forth or left and right, versus a clockwise or anti-clockwise turn at the end of the line).
Pendulums are not so great for telling the future, but can be useful for whether something is more or less favorable, or the state of something.
Some people may attempt using them to spell things out, with longer or shorter processes, to locate items, and so very much more. There are many, many charts that people have made. And of course, you can always make your own.
Use discretion. Know what works for you already, and use (or design) any charts you may use to be compatible. For example, if you're yes-no is a clockwise or counter clockwise turn, don't use a chart for a left-right or back and forth swing.
I do suggest finding at least a beginner's book and going through some exercises, again using your own discernment whether something in the book is something you would want to do with a pendulum.
Something a pendulum can very well be used for is a connection to your subconscious mind. That's partly why you are better off clearing your mind before use, and not to ask questions about which you feel strongly. If you ask whether you should ask that special someone out, your subconscious may give you a strong YES because you have an affinity towards that person. But if you're trying to remember where you left your keys, a pendulum may help with yes-no questions (or running over a floor plan quickly drawn out) since your subconscious never forgets those things.
Use of the pendulum for things outside yourself is where things start getting freaky. It's when a pendulum is used for things like dowsing.
Ultimately, a pendulum can give very limited immediate answers. That dictates what kinds of inquiries can be made, but also what makes it so versitile. Again, I suggest finding an actual book. Members here can give some decent recommendations, and I know there are several in the SOM shop. It's where I bought the book I used.