I have been told that you start feeding when the flowers start to turn into tomatoes as if you feed before, you encourage leaf growth.
Was also told that tomatoes have two sets of roots, the ones near the top take up feed and the bottom ones water. So apparently it is better to water from below and feed from the top.
Some people on a forum I've been reading get a growbag and some largish pots which they cut the bottom off and "plant" in the grow bag and fill with compost, with the plants in the pot. A cut off squash/water bottle gets put into the growbag and the watering goes into there, then when feeding starts it goes into the pot at the top.
Apparently they need a certain temperature to grow, if it is too cold they stop growing. Apparently you plant them out when you can see their first cluster of flowers but you'd need to start hardening off before then.
My plan is that I'm going to start hardening things off beginning of next week and aim to have the courgette (huge and flowering on the kitchen windowsill) and tomatoes out at the end of the month as I don't trust there not to be a late frost and kill all my precious seedlings (currently more cossetted than the DCs )! My aubergines are in my little plastic greenhouse, growing well and will stay there, though I have read they can be quite hard to get to crop.
Have also read that you can let the size shoots of a cordon variety grow a bit then pinch them out and whack in compost for new tomato plants.
I do need to stress that all the above is what I've been reading and my pathetic attempts resulted in about 2 tomatoes last year so am not the person to be giving advice, but all this came from forums where they seem to know what they are talking about !