Hi Ms. Astro Forever,
Gosh...
You asked such a simple and great question. It's difficult to be brief, but I'll try. Yes, you can think of the rhymes first, then write the lines around them. I often used to do that, and for variety, I sometimes still do. In this way, it would be like writing an outline before writing a paper. And it really helps to use this method if you're new to writing rhyming poetry.
Once you become more comfortable with how to rhyme words, and more comfortable with writing rhyming poetry, you'll be able to write it as you would any other written form. There is a power in words, and magic in poetry, but magic is only a science which isn't understood, right?
It's clear you already know how to communicate and write well by the depth of articulation in your posts here in the forums. I think the trickiest part of writing rhyming poetry is actually the rhyming part itself.
If you learn how to rhyme words of any length or rhythm, writing poetry merely becomes a mental exercise in recording your thoughts just as you would any other.
These are my theories. I could also offer tips, but I don't want to spoil your discovery or learning processes as you find your own poetic voice, unless you really wouldn't mind that kind of help.
Cheers!
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Thank you so much!
It would clash here, with all those great pieces Novacain and yourself are writing! 
