Follow the instructions on the seed packet & you shouldn't go far wrong.
Try some of the cheap copper scourers you can get from the cheap shops like poundland. They look like this>> click for pic of copper scourer.
If you unravel one you can re-wrap it around the base of plants to deter slugs & snails at ground level. I did this with brocolli, and, touchwood, not had an attack since.
Carrot fly won't fly higher than 20cms from the ground, so as long as your tubs have sides taller than 20cms your carrots will be safe.
Carrots prefer sandy soil too, with the stones sieved out if poss.
Too much composty humus will cause carrot 'fanging' like this >>> rude carrot
Last year I successfully grew parsnips for the first time in a garden (or builders) trug. They look like this>> a purple trug
and you can get them in all colours & sizes.
Remember to poke holes in the bottom for drainage though.
I got mine from Wickes, I think they were £2.99 each & will last for many years!
Hope this helps?