Thumbnail Problems Please help

Hello,
I am fairly new to wordpress and am trying to create a tube site to display video galleries and such. Here is my problem I know to make a post with the video embeded into it and the post shows up on my themes front page correctly the problem is it wont display a thumbnail of the video and I cant figure out how to get it to work. I have tried several plugins but nothing works. Alot of places tell me I have to add certain lines of code to my function.php file but everytime I do it screws up my site and I have to redo it all through FTP. Can anyone tell me how to do this as easily as possible because I have been trying to figure it out for almost 3 hours now and im about to rip my hair out lol.

Thank you in advance for any who can help me.