If you are a novice knitter, the standard long grey needles will be fine & size for DK yarn will be determined by what you are knitting - usually between 3mm and 4mm. The pattern will state what size.
Worth looking at Ravelry for project ideas and many patterns on there are free. Also consider companies like Wool Warehouse who are very reasonable and offer a huge range go yarn, needles and accessories. Again, they also have ideas for projects. If there is a Baby Bank near you, they will be very grateful for simple baby blankets which don't take long to knit. Good luck, I've been knitting since I was 9 and it soothes my soul. More recently I've learned to crochet (from YouTube) and I enjoy that too though I'm less proficient at that.