Hi Karen. Lucky you
It dependes on just whereabouts you are and what the seeds are. Some things such as tomatoes, chillies, peppers, cucumbers and so on are very tender - which means that not only will they die in a frost, but also if the temp falls below a certain degree, which for cucumbers and chillies is about 14. So it's best to sow them in a warm place indoors, keep them potted on into slightly larger pots each time they fill the old ones, then as it comnes close to the last frosts for your area, harden them off (by putting them in the tunnel in the day and bringing them in at night,) before eventually planting them in the tunnel.
Other things which might not be ok outside, such as lettuce and radish, will be fine planted direct into your tunnel.
Sounds as if you need a good veggie book to go with your new tunnel so you can make the most of it