Help with syncing on itunes?

so i got an ipod last week. its not the ones wit a camera. so i downloaded the latest itunes. so i have like 100 songs on itunes but i cant sync them! at first i was able to cux it did it automatically but i changed it to manually so it wouldnt sync my whole library evry time but now i cant sync the songs that i add into my ipod. i set it up so that i could sync the songs that i selected but it wont sync them & when i try to it deletes songs from my ipod and now im stuk wit only 20 songs on my ipod & cant sync the rest! so can sumone tell me what to do, cux im tired of listening to the same songs :P plz & thank you :D