Many of the tracks are rough and steep, so a good pair of boots is essential - certainly not flip flops of light shoes.
Consider taking a map and compass, a waterproof coat and over-trousers, warm clothes a packed lunch, water.
Several thin layers are better than one thick layer,
For longer walks, you might need a hot drink, whistle, a torch with spare battery and bulb, a woolly hat and gloves in Winter especially, hat, sun cream and sunglasses.
Many people trip on uneven ground - and there's a lot of that on Snowdon, even on the gentler paths - so a small first aid kit is a great idea.
Make sure you know the route - not just up the mountain, but down again. If you come down a different route make sure you know how you'll get back to your car or hotel. There are regular buses around the mountain but even they don't run at night .
Try to let a friend know your plans, and check in on your return.
Some parts of the mountains have no phone signal - which means GPS may be patchy!
And don't forget a camera!