You will make things far easier on yourself if you start with a nursery grown plant. Rosemary can take some time to fill in as a plant, so expect to pay more for a mature plant than for a small rosemary start.Rosemary is usually propagated by cuttings. Seeds can be difficult to germinate and often don't grow true to their parent. It's much faster to start with a cutting and you will be sure of what type of plant you will get. It's possible to root rosemary in a glass of water, but a bit more effort will give more dependable results.