Here's another conundrum I spent many hours figuring out. How can I generate a vocabulary, or lexicon, without it taking YEARS?
There are a few different schools of thought on this. Some people feel that each word needs to sound like what it is, within the confines of their phonology. Meaning, you think about and create each word. This is very abstract, but you just might come out of it actually being able to remember a lot of your words, maybe even be able to speak your conlang (Remember, VERY few conlangers are fluent in their language, and the ones that claim to be are suspect, because who can really judge them?). Plus, you're guaranteed to get a conlang that sounds the way you want it to.
The opposite extreme is to randomly generate your vocabulary, after keying in your phonology to a word generator program. The advantage is you get a big vocabulary quickly, the down side is that you won't know any of the words off the top of your head until after some studying, and some of the words may not be to your taste.
I started out wanting to randomly generate my lexicon, but found LangMaker and word generators like the one linked above to be inadequate, at least at first. I had quite a time figuring out a good word list to use; I started with Ogden's Basic English, which has about 850 words. However, it is a list generated for teaching basic English, not for creating a conlang. Some words in the Basic English list might be "covered" differently in the word list of another language.
But I found another list that I thought was better, mostly because it was much shorter: the Swadesh List. Only about 200 words there!
In the second Language Creation Conference, John Clifford spoke a little about semantic primes, which aren't "words" so much as they are blocks of meaning. Its a different way of thinking, but a little reading here can also help you develop a word list of your own. I found a word list, called the Universal Language Dictionary, that groups words together according to concepts, which may help you if you want to create a derivational morphology or something. The ULD at least partially embraces the semantic prime idea, and can be another good resource for developing/building/copying a word list for lexicon generation.
So, with a short list, you CAN use the first abstract method, or you can randomly generate, and then change words as you determine better sounding ones, and add to the lexicon as you translate phrases. Long lists may be more cumbersome, but can be worth the time and headache if you plan on doing a lot of translating, as you won't have to stop to create a lot of new words each time.