I just finished installing a new kitchen faucet. There is a slight leak coming from between thenut that tightens onto the faucet and the metal piece between the plastic hose and nut. I tightened it more, but I can see it's coming from between the two above-mentioned pieces. Do I have to get a new hoseline, or can I fix it with something?

If the unit is new and you installed it you should reread the installation instructions and be sure you follow every step. This may require you to uninstall the unit. More than once I thought I knew how something went together but later found out I skipped a step or did something out of sequence

If this is the setup I think it is, it is hard to get in there and tighten it, yes? I really think you just need to get the right tool in there to better tighten the hose connection to the bottom side of the faucet. A basin wrench, or small cresent may do the job.