Now that you have the sounds that your language can make and how they interact with each other, you can begin to come up with words.
How many words do you think you need in your conlang? Well, a 2013 (admittedly not super scientific) study revealed that adult native English speakers generally know somewhere between 20,000 and 35,000 words. So get cracking!
Now, if that number seems daunting, there are a few rays of hope. First, the same study found that non-native speakers who lived in English speaking countries generally know about 10,000 English words. So 10,000 is probably enough for most day-to-day purposes. But that number is still pretty big. How can you come up with 10,000 words that sound consistent but also different enough to be clearly distinguishable? The answer is: you don’t.
That 20-30k figure is not counting distinct meanings, but distinct words. So “happy” and “happiness” were counted as two words, even though the only difference in meaning between the two is how they are used in a sentence (happiness being a noun form of the adjective happy). When you’re coming up with your language, you should take advantage of this. Create root words and then figure out how to modify them for use in different situations (i.e. parts of speech) and variants of meaning.
There are a few ways you can build words once you have the building blocks to do so. One of the most common is to use affixes. Two kinds of affixes that English speakers should be familiar with are prefixes and suffixes, but in case you haven’t heard the terms a prefix is something like “un” which we can stick on front of a root like “happy” to make the word “unhappy” while a suffix is the same thing only applied to the end of the word (“ness” in the previous paragraph). What English speakers tend to be less familiar with is infixes, which is an affix stuffed into the middle of a word. This is not common in English. In fact, I have only ever heard one infix in English as far as I can remember and its use as an infix is a rather recent addition. It can be seen in the word “fan-f-ing-tastic”. There’s actually even more kinds of affixes than just these three and I would highly recommend you take a look at the list Wikipedia has. It gives some good examples.
So come up with root words and affixes and you can start constructing all sorts of words and get well on the way to that 10 or 20 or 35k (or more) lexicon.