Hi MTL,
"To reach a decision" - to consider something and then come to a conclusion.
They discussed a number of options, and reached the decision that option A was best. Everyone agreed that they had made the right decision.
Amongst other things, 'form' can mean "to make something begin to exist:"
"To form a conclusion" They didn't know where to go, but after discussion they began to get ideas which helped them to come up with a decision (which had not previously existed)
English can be quite difficult to learn, because we use one word to mean many different things.
You might find this on-line dictionary helpful, as it gives examples of sentences using the word you search for.There are 6 disctionaries here, including alearners and advanced learners dictionary. You can spend hours just reading it!
http://dictionary.cambridge.org/
Here is a good on-line dictionary