Here is a site where you can find out your gardening zone.
And here you can find out your average last frost date. Some things can be planted before this date. Read your seed packets to see what it says for each type of plant.
For our natural dye garden, herb garden, and vegetable garden, we've planted lots of seeds inside and now they're big enough to move outside. Hooray! But we don't plant them right away. The seedlings are used to a fairly consistent temperature from being inside the house. They haven't felt much wind. The intensity of the sun's light and heat has been buffered by walls and window glass. They were watered regularly and were never drenched by rain or parched from a hot, dry time between waterings.
To help seedlings acclimate to living outside, sit them in a protected outdoor place for a couple of days. I put mine on my back deck, right against the wall. The house blocks some wind, sun, and rain. The plants are close enough to the door that I can easily bring them inside if it gets too windy and they might blow away.
Ideally, we'd plant on a day when it's cloudy and even misting a little bit. If you live in the Pacific Northwest, good for you! You might have a drizzly day right when you're ready to plant. The rest of us will have to do our best with the weather we've got. I recommend having your hose or watering can ready so you can water right away. If it's sprinkling, you may not have to water that day.
If you make straight rows, you'll be able to mark them and easily tell what's growing where. This style of gardening makes it much easier to tell which little plants are wanted and which ones are weeds!
If you don't worry about straight rows, you might be able to fit more plants into the garden. Some plants can go closer together if you give them enough food and water. I often plant in a messy, crowded way, and it works okay for me. For instance, I might plant my tomatoes in nice, neat rows, but then stick basil plants in between them. If you want to plant closely together, I have a few tips.
One thing to think about is choosing different heights of plants to be next to each other so that they all get enough sunlight. A great example is the Native American tradition of planting corn, beans and squash in the same spot. The corn grows up very tall, so nothing blocks it from getting plenty of sun. The beans wind around the corn stalks without harming them, while also getting full sunlight. The squash stay low and get plenty of light as their leaves are wide and the corn and beans don't cast much of a shadow.
This combination is also wonderful because corn needs a huge amount of nitrogen and beans add nitrogen to the soil. Adequate nutrition can be provided by feeding well on a regular basis, but it's so nice when the plants work together!
Other things to consider when choosing plants to go together are insects and disease. Some plants help protect each other by emitting a smell that's offensive to insects that would eat the other plant. Some plants attract insects that help the other plant by eating aphids or by pollinating the plants. On the other hand, some plants are in the same family and they can spread diseases to each other, so it's better to keep them separated.
There are a few books on companion planting that I love:
Tomorrow I'll tell you how I'm laying out my garden beds, with friendly plants next to each other.