Advice for first demo?

im a singer/songwriter putting my first demo together with garage band does anyone have any suggestions?

what kind of tune you looking at doing? like uh, what kinds of instruments do you want to use in all;drums, bass, guitars(acoustic? electric? both?), vocals, piano/synth effects, etc.
If its going to be a pretty straight forward acoustic and vocal song only I would say experiment with placing a microphone in different positions around the guitar(wherever sounds best to you) and the vocals are pretty straight forward, just belt em out on a separate track(or on the same if you really want to).

As far as arrangements, that is all you, so I won't touch it.

Now, for the mixing and such, again that is all dependent on what you want and what type of music you want to make.
Playing around with a little bit of reverb on the acoustic guitars(only a tad) can really be fun and make a totally different sound. Double and even triple-tracking guitars is a lot of work, but your overall guitar sound will be thick and extremely awesome(also harmonies can be done with a second guitar).

Basic thing though man, just go for it! Do what sounds best to YOUR ears and go with the takes you are happy with.

I've worked with Garageband before(albeit it was about 6-7 months ago) but if you have any questions on it, I might be able to help, so just ask away. This a very helpful community, so don't feel bad to ask questions man

Most of the songs i plan to record include a garageband preset drumkit, multi layered guitar tracks(some acoustic and some with effects), vocals, and some sound effects to contribute to the mood of the song. To label the sound id say my songs have an alternative/progressive acoustic rock feel. i have been writing, playing, and developing my own songs for about five year now but have never recorded. well iv decided to make it my career so now i am in the process of collecting all of my songs together, finishing them, and recording a demo to start.