If you're going to make a mobile out of wires there are a few tools that you will need to have. The round nose pliers are necessary for making loops in the wires, and flat nose pliers are for making adjustments and just holding onto the wire without damaging it. You should also have a wire cutter and a pair of needle nose pliers for all sorts of adjustments. Here are some affiliate links where you can get some pliers on Amazon, although you may prefer visiting a store and holding the tools in your hand to see how it weighs and feels.
You can get a simple set of tools when you're first starting out, but with time and experience you'll realize that certain tools need to be replaced by better quality tools. You'll need a good set of wire cutters - if you're working with steel then make sure you don't buy a cheap one. I used to struggle to cut wire with the cutting edge of my needle nose pliers, so I finally bought the right tool for the job. Your round nose pliers might also have be replaced for one that's more heavy duty - where the tip won't bend when you work with thicker wire.