Sweet peas are usually started from seed. You may either plant them in seed trays indoors and later transplanting them to the garden bed, or start them outside. Seeds are available for purchase at any garden store. For rarer varieties, check online retailers.   "Old-fashioned" sweet peas will produce very fragrant flowers. Spencer varieties have bright colors, but less fragrance. You'll find them in pink, purple, blue, white and red. Sweet peas can be grown in any growing zone, but it's important to know the right time to get them ready. They must be planted as early as possible to form a strong root system and survive the summer. Therefore, starting seeds early in the year is usually your best bet.  If you live in a temperate area where the ground does not freeze in the winter (USDA zones 8-10), you can plant your seeds directly in the ground as early as November, though waiting until January or February is also fine. Be sure to water them over the winter, and they'll emerge in spring. If you live in an area where the winters are cold, starting seeds indoors is your best bet. That way, the seedlings will be ready to plant as soon as the first frost has passed. If you wait too long to plant your seeds, they won't have time to take hold in the soil before the summer weather gets hot. Another option is to sow seeds directly in the ground in late winter, and then allow them to come up when they are ready. Sweet pea seeds have the best chance of germinating if you help penetrate the seed shell before planting. You can do this by either soaking them in a pan of water overnight, or using a tiny knife or nail clipper to nick the surface of each seed. If you soak your seeds, only plant those that have swelled during the overnight soaking. Discard those that haven't changed in size. About 5 weeks before the last frost (usually mid-February or so), prepare small seed trays or peat containers with a seed starting mixture. Plant the seeds 1 inch (2.5 cm) deep and 3 inches (7.6 cm) apart, or in separate compartments. Water the seeds trays and cover them lightly in plastic wrap for the first week or so, to regulate the temperature. Keep them in a greenhouse or in a sunny window in a place where the temperature won't drop below 70 °F (21 °C). Once the seedlings sprout, remove the covering and keep them moist and warm until it's time to plant them just after the last frost.  If you're using seed trays, thin out the seedlings to space them 5 inches (12.7 cm) apart once they've sprouted leaves. Pinch of flowers and buds before transplanting, so the seedlings' energy can be channeled into growing new roots.
Buy sweet pea seeds. Determine when to start your seeds. Soak or nick your seeds. Plant the seeds in seed substrate. Keep them moist and warm.