I failed to melt chocolate bars in order to make chocolates in mold. I tried twice using water bath and microwave but it wasn't as runny as I had expected but much like paste. When I continued heating it, it was broken, becoming like cookies dough. Was it due to the type of chocolate I used? By the way, what should I do with broken chocolate? Thank you for your help.
The dough-like texture you mention makes it sound like you burned your chocolate. Remember, a bar of chocolate will melt in your pocket so it really doesn't require much heat to melt.
To use a microwave, break the chocolate into small pieces and stir every 30 seconds even if the chocolate is still solid since the microwave will heat the pieces unevenly; it shouldn't take more than about 90 seconds in total.
Using a water bath, make sure that the bowl is not in contact with the boiling water (to be honest once the water has boiled you should be fine leaving the bowl in place and taking it off the heat, letting the residual heat warm the chocolate).
Additionally, I would recommend 'How to Cook That' and its YouTube channel for good clear videos about how to melt and then use chocolate. In particular if you want to mould your chocolate you will need to learn about tempering it, and the guide on that YouTube channel is comprehensive and reliable.
Normally, couverture chocolate (real chocolate) is used for molding. However, I've actually seen compound chocolate (once) successfully used for moulding too.
Regarding what happened to you, there's the possibility that 1) water/steam got into the chocolate while melting, causing it to "seize" (check the internet for "seized chocolate" 2) you were using compound/imitation chocolate (basically cocoa powder mixed with some vegetable oil) 3) overheating (chocolate should not be heated above 50 C. In my case, I normally just heat chocolate to around 45 C) or 4) a combination of the above.
Broken or seized chocolate can still be used for baking and other such recipes.
Note on why couverture chocolate (more expensive) is different from compound (cheaper) chocolate:
Compound chocolate is basically imitation chocolate. It's basically cocoa powder mixed with some form of vegetable fat (palm oil, vegetable oil, etc). Couverture chocolate however is made from cocoa/cacao beans and cocoa butter.