Post your songs to this forum and people can download the songs and mix them for you. Then the person who has mixed the song can post it in a comment. If we get enough members, every song that's posted will have a few people mixing it and then the author can choose his/her favorite mix. You can even do a sort of "remix" of a song in which you do a techno version of a song or something fun or bizarre like that.
Feel free to do other things, too, such as ask for an opinion on your song or ask questions about music theory or music equipment or anything related to music. This community also encourages that you upload, for example, the guitar part and vocals to a song you have written and then ask "someone, please, overdub drums" or "someone overdub a bassline" or "someone overdub whatever they want (piano, guitar, loud honking sounds)".
We can also set up songwriting contests (or cover-song contests for the best rendition of a song). All styles of music are fine--rock, metal, jazz, blues, foreign, country, rap, classical, punk, easy-listening--I'm sure there will be someone in the community who shares your interests.
Anything's fair game really, just utilize the community for whatever you may want to use it for.