It's not bad, and you've got potential.
1. Toy around with graphics to try and get some better looking ships.
2. Speed it up a bit and put in a mission progress bar.
3. Throw in some power ups.
4. Find some better music, and maybe work on the AI's attack pattern so it's less predictable, orat least a little harder.
5. make the current power-ups more distinguishable from the ground, and maybe make the things on the ground destructable. Sicne that's at first what I thought the nukes were for.
I didn't play for long so I'm not sure all what there was, but some of these should stick regardless. Goodluck and keep practicing.