Pruning tomato bushes is a tradeoff. If you prune the later fruit
(flower) clusters the earlier clusters will ripen sooner. However, the
total yield from the plant will be reduced. If you have several plants,
you might prune one or two and leave the rest for production. Pruning
will not initiate flowering or fruit production.

When pruning, you don't cut the tips of the vines. Look for the main
stem. It will branch out occasionally. At the branch, a leaf will form
between the branch and the main stem. This leaf will eventually turn
into another vine which will bear fruit. However, the point at which it
joins the stem is weaker than the main branches and these vines are
subject to breaking off. These vines are the ones to prune. The easiest
way is to just break off the leaf that forms at the junction. They are
generally brittle enough that you can just bend it enough to break it
off, and you don't need pruners.

If you do enough work in your tomatoes your fingers will turn black. It
will wash off, but even when your fingers look clean, washing your hands
will turn the soap yellow or green for a couple of hours. It's just a
thing that tomatoes do, and doesn't really hurt your hands or the plant.

The best way to choose tomato plants at a garden center is to look for
good color and no wilting and the thickest stems, not the tallest
plants. Tall skinny tomato plants have probably been crowded together in
their growing area. They will recover once set out, but it will take
them a bit more time to bear fruit (possibly a week or two depending on
how crowded they were).

More sun is better, but the plants will produce with 4 hours of sunlight.

Frank wrote:
...the tomato plant grows to almost the tallest ring but I
haven't got any flower not saying fruit. I'm wondering whether I
should prune the plant, ie cut some tips? When, where and how? My
plant may only get 4 hours afternoon sun. It is the best I can do with
my location. How bad will this affect the flowering and production?...