I'm pretty clueless, but you find things out. And if you get it wrong, you can try again. There's always next year.
There are so many books, but the one you found looks ok. I think he's on GQT.
The seed packets have instructions. You can use a bit of leeway with the sowing dates for a lot of seeds, but not for things that need a long season. A mistake I do is sow things too thickly.
I plant the end bits of spring onions, leeks, spring greens, cabbage and lettuce, and tops of root veg. They usually grow into new plants. You can sow herbs from the spice jars in the kitchen.
You can sow seeds from fruits - I've saved seeds from squashes and had successes. Squashes are promiscuous - you might get strange hybrid fruits if you have different types of squashes. They will be edible but a bit odd-looking. I think you need to exercise some caution if you are also growing gourds as the fruits are poisonous.
Some websites do a what to do in your garden this month newsletters.
If you are buying tools, you don't need many. I like a stainless steel trowel, and I love decent secateurs that fit my hand.
If your greenhouse is in a sunny place you might need shade for the midday in summer.
The soil you sow seeds in is best not too rich. Add fertilizer or feed after the seedlings are established.
You'll probably want a compost heap or bin.
Put something over the ends of bamboo canes, or wear safety glasses.
if you plant bulbs like onions, make sure they are all under soil.
Birds will help themselves otherwise.
Wilko is good for reasonably priced seeds.