You can quickly join a series of lines onto a single line using the J command. Simply hitting J in normal mode will join the current line with the line below it leaving a space in between the two. If you want to join the next 5 lines, you can hit 5J. Hitting 5J on this:

first,
second,
third,
fourth,
fifth

will transform it into this:

first, second, third, fourth, fifth

Similarly, you can perform a line by line join on a visual selection by hitting J after making the desired visual selection.