When you plug your iPod into your computer, it should appear under iTunes under the Devices section. If you click on this, on the Summary page, do you have "Manually manage music and videos" ticked? If not, if you go on to the Music tab, from here you can select music should be synced and then choose whether you want everything synced or just selected playlists. If you don't have enough room on your iPod, you can just create a playlist with the tunes you want on your iPod within iTunes and then tick the box for this playlist. You should then be able to Sync successfully.