te = you lo= it / him te digo = i tell you lo digo= i tell him no lo tengo= i don't have it

te is occupied in phrases like this: Te amo (i love you ) , Te necesito (I need you ) , Te quiero (I want you ) . And "lo" is use like this : Lo amo ( I love him) , Lo necesito ( I need him), lo quiero (I want it )

It depends of the context. For example, the translation of "Te lo dije" in English is "I said it to you". "Te" is the part of the oration which makes a reference to "to you", while "lo" makes a reference to "it".