I would be surprised if a pattern didn’t state which needle size to use. Knitting is the same as crochet – needle size has a great deal to do with what size an item will end up being. For just practicing it probably won’t matter too much as long as you’re using an appropriate yarn weight, but for an actual pattern you will want to be careful of needle size and gauge.
If you just want to get started practicing, I’d suggest a size 8 (US sizing) needle and a worsted weight yarn.
As far as straight vs. circular…a lot of people here knit everything with circular needles whether they are knitting in the round or not. Circular needles can be used for knitting things flat back and forth, turning the work around at the end of each row as though you were knitting with straights.