If you're doing the Llanberis path I did it in a pair of trainers. I thought the first mile or so was the hardest, after that the majority is a slow incline. Take some high factor suncream and apply it regularly because it might not feel warm halfway up if it gets a bit windy but you'll be ravaged by the sun (I found out the hard way).
Take some snacks and a big bottle of water- there's only a hut halfway up and at the top.