I am kinda stuck when I try to make something...
You don't say what you're stuck on. Or what problem you're trying to sort out.
.. my only complain is that it doesn't take mp4 format which is pretty standard in the industry
Yeah, but you don't score or create your music around the full resolution version of your video. You should have a low resolution, smaller video to create your music around or time your sound effects to. In that case the format doesn't matter. Make one for whatever FL uses. And you adjust the music and the video to work together. Then when you are done sound mixing you import the finished track into your main video editing program.