Spend a lot of time with the beat you're trying to rap over, internalizing the sound and the rhythm of it, to find your flow before you start coming up with lyrics. Like you write the melody in a traditional song first, you have to find the flow first in a rap song.  Some rappers will do a similar "nonsense word" technique, just spitting rhythmically without saying actual words. Try to record yourself doing this, even if it sounds silly, because something good might leap out. Good rapping is as much about flow as good rhymes. If you stay on beat, it's better than if you lose the beat and Try to force awkward or overly complicated rhymes into the structure of the song. Like you might do a freewrite to start getting poetry out, trying some freestyles is a good way to get started and find a starting line to use for a song. Or, if you're Riff Raff, just record your freestyle and call it a song. There's no rule that rhymes need to come at the end of each line, especially in hip-hop, or that the rhyming word needs to be the end of the sentence. Vary the placement of the rhymes. Embed rhymes internally and skip rhymes entirely to add variation to your flow. You don't have to rhyme at the end of each line to rap well. In "Duel of the Iron Mic," GZA creates a particularly strong break in the lines, using a well placed and surprising break in the beat to surprise us: I ain't particular, I bang like vehicular / homicides, on July 4th in Bed-Stuy Familiarize yourself with the greats, listening to a wide variety of rhymers to begin learning the craft. Listen to:  Nas, who jumped on the scene as a teenager with his classic album Illmatic, which featured these lines:  It drops deep as it does in my breath / I never sleep, cause sleep is the cousin of death.   Eminem, whose intricate and well-crafted rhymes have made him a bona-fide king of the rap game:  I'm Slim, the Shady is really a fake alias / to save me with in case I get chased by space aliens.   Rakim, one of the most influential MCs in hip-hop:  Even if it’s jazz or the quiet storm / I hook a beat up, convert it into hip-hop form.

Summary:
Listen to the beat and find your flow. Freestyle. Learn and use enjambment to your advantage. Listen to expert hip-hop rhymers for inspiration.