Some conlangers really enjoy building a lexicon piecemeal. This method creates a rich vocabulary where sounds and morphemes are all hand-picked. These lexicons have the advantage of highly customized, original etymologies.
Sadly, I am not one of these people (yet). I, more or less, just wanted a vehicle for writing pseudoglyphs. The lexicon was generated randomly—with little etymological attention—using some amazing software.
LangMaker can generate words according to patterns of consonants and vowels you specify, can transform words according to sound changes, can derive words from etymologies, and much more (read on).
The wizard has some good pattern generators.
Or create your own rules.
The drawbacks are obviouse: pre-assigned parts of speech, pre-assigned compounds, an English etymology. Still this is a good place to start.
This is why the ‘Umu word for cat is ‘a’a and the ‘Umu word for pig is ‘u.