How to Write Music to Go With Lyrics?

[Strawberry Gashes]

Hello! So, I'm an aspiring singer, and I've got no trouble playing instruments. The problem is, I'd like to sing original songs instead of cover songs, but I can't write music. I can write the lyrics, but sheet music is definitely not my strong suit and I can never put the melody in my head onto a page, or play it on anything. My question is... how to write music? Like, how to put chords on a keyboard or guitar or whatever else together to make a melody that I want? I just can't seem to get it right, or how I want it at all. I can't download any software, since my only recording device is my mom's old computer, and I can't purchase any sort of guide... really I'd just like some advice.

Thanks in advance!
Strawberry Gashes