Nobody has responded so I’ll answer, despite not living in a place where watermelons can be grown.

First of all, the seedlings are etiolated ie they are long and thin and pale due to lack of light. They need to be gradually introduced to brighter light, eventually being set out in full sun.

Secondly, do you have garden space? Watermelons grow enormous long vines and need a large area to grow in.

And lastly, although I have no experience growing them, I believe it is too late to plant them out in the garden and get fruit. Unless, of course, you live in the Southern Hemisphere or a year round tropical climate,