Spinach is not a commonly grown crop. Most people grow leaf beet, commonly called spinach. It is also readily available in bags as a salad leaf, sold as baby spinach and almost always imported. I think you would have to be bordering in insanity to buy fresh spinach to cook because it famously cooks away to nothing. You can fill a saucepan to the top with spinach (and the same applied to leaf beet) and after a few minutes cooking it is a pathetic green lump at one side of the base! But I am not trying to put you off growing spinach.
Spinach is a cool-growing crop. It likes moist, cool growing conditions. If the weather is hot and dry the plants will quickly run to seed (see above with the variety ‘Bordeaux’ which is unusual for its red stems). This is not a huge issue because you can eat the shoots as they bolt, but it does mean that the useful life of the plants is over. I know I prattle on about sowing for succession but it really is worth sowing little and often. I would suggest sowing from March to May and then again in August and September but you can stretch the sowing time if growing in a polytunnel. Lots of modern kinds are resistant to mildew though I have never had that problem.
Because spinach is a short-lived crop I would pick the first leaves as salad leaves and as the plants start to bolt pick for cooking and if you miss the sweet spot and they get stringy you can juice them. Spinach reaches picking for salad stage very quickly and is a good patio crop especially as it will grow in shade. It is well worth a try.