I use a rating system to help me decide what to grow and it considers lots of factors, but the main ones are how tasty it is, how healthy it is to eat, how expensive it is to buy, how big it's harvest is, when it's harvest period is and whether I can buy it organically and if not how much it's sprayed.
Celery doesn't score particularly high, so we only grow a little of it, but we harvest it as a cut and come again crop for up to 12 months, so a small area is very productive.
The way that I grow celery is very atypical, if you want to know how everyone else grows celery, take a look at the RHS web site.
I like to sow celery in early February, so that it's ready for planting in early April. I only sow a single batch and we will freeze some too for winter soups etc.
I sprinkle seeds in a small plant pot and cover with a 1-2mm of sieved compost and water lightly. I then cover with a polythene bag and place on a north facing window sill, close to a radiator. Patience is needed as germination takes a couple of weeks. If I forget I have been known to buy a small pot of seedlings from a garden centre.
Transplant the young seedlings when large enough to handle, which might not be until several true leaves appear. Tease apart the roots and plant one plant per cell in a six cell tray.
Keep the seedlings above 10c until they are well established and ready to plant out, otherwise they are likely to bolt (bolting early is usually a response to stress). Keep them with your cucumbers, tomatoes and peppers.
For more details on how I sow seeds and all of the products I use, see my guide to sowing.
Celery, planted in a polytunnel for continuous harvest
Celery likes moist soil and it hates to dry out, but it doesn't need any special bed prep beyond compost rich soil. I plant mine in my polytunnel where it really thrives in mid-April. I drape a little fleece over it if frost threatens, which it often does, but I've never lost a plant to the cold.
In mid April I make holes with my hands and drop a module in every 9". I plant closer than recommended (12") because I keep my plants small by continuously harvesting one stalk at a time. Twelve plants is enough for a handful of celery a week.
Some people make a big deal about blanching the stems, by wrapping with card or earthing up. We've never found this to be necessary and everyone seems more than happy with the taste.
Although everyone seems to be obsessed with how much water celery needs, we don't over water it. We just plant it like every other plant and water it just like the tomatoes that grow adjacent to it in the polytunnel. Don't let it dry out though.