Does anyone have any ideas on what I can make as my first (actual) map? I don't want any old Aperture suggestions, I'm not up to that stage when I can make that stuff yet, but I can make decent chambers.

Also, if you would like, feel free to suggest some tips on how to add detail, where stuff like that can fit and stuff like that. Thanks.

pick 2 elements... put them in a room, maybe add some gel, now add a cube and buttons that control those elements. Now use portals and the first element to achieve the second then use the second element and the cube or the first element to get the cube to a third destination...then use all the elements to get you to the final destination, yay! or something like that

Edit: LP I think I submitted my post mere seconds before you!

To come up with puzzle ideas is mostly creativity, there are different methods every mapper uses and there are a couple of them mentioned in this thread mapping-help/puzzle-producing-process-t5983.html.

As for puzzle design: that's really up to the mapper. The creativity that is used for the level design is something that can't really be learned. Though you can look in the official portal 2 maps and/or maps here, note what you like and try to use that for your maps.

I would recommend making your first map a rather small one and release it to the WIP section. Here you can get advice from others.

I suggest to play other video games for ideas. Some worked out pretty well for me.