First up, if you want to output to a computer, as Tom says, always shoot progressive.
But you have video that is interlaced, and you want to make it as good as it can be.
If you just want playback on a mac or youtube, and you've shot in 1080i, you can click the project in FCPX in the Project Library, then at top right in the Properties panel click the spanner icon at it's bottom right, then change the video properties to 'other' and 960x540 and frame rate, so if it was 1080i60 (30 frames/sec, 60 fields/sec) then choose 30 fps so you end up with 960x540 at 30fps.
When you export using project settings, the result will have NO interlacing at all. There will no blurring at all on the edges (the blurry line edges just won't exist), and the stills will look perfect, but of course you've lost some resolution, and it will be a bit more stuttery because the blurring actually makes movement easier on the eye.
BTW add about 15 of edge sharpening in compressor and the final output looks amazing.
But you've effectively only got a half-size output.
Note that the original video is still there, so you can change the options back to the native format and export as 1080i if you like.
If you want to try to retain full 1080 info, and if the lens itself is sharp enough to warrant it, and you don't care that the file size is big, then you'll need to do some deinterlacing.
The best way is to purchase Compressor, figure out how to use it, and use it's high quality deinterlacing options to output a 1080p file. If you shot 1080i60 you will end up with 1080p30. This will give you good results - no horizontal jaggies - and retain all the detail where there is no movement. However, where the horizontal lines currently exist on moving edges, these will be replaced with a blurry region. That region will look a bit 'unnatural'.
There are quite a few deinterlace options, so you could also try a 'single field' option. If you choose that, Compressor will use one field and stretch it vertically to fill the frame, so that you get the full horizontal resolution but only half the vertical resolution (every second vertical line will be the same). This way there will be no blurry edges at all, but one entire field each frame will be discarded, so while the horizontal detail won't change, you lose half the vertical resolution.
So you can experiment with the different deinterlacing algorithms choose whichever you think looks best for your source material, but no matter what you do, you lose some information or gain some blurring - the result just won't be as good as if you had shot 1080p30 to start with.
1080i video is intended for export to blueray for playback on a TV. No matter how you try to make interlaced video PC-friendly, you're going to lose data or get annoying artefacts of some kind. Interlaced video is for TV's, not computers.
If I have to work with 1080i, and all I want is a movie to play on a computer, and I want a compact file size, is to set the project properties at 960x540 and export with a little sharpening via compressor using the x264 codec at that size. I find that if I deinterlace and try to make 1080p out of it, I get a marginally bit better quality, but not a lot. Of course if your video source is really sharp, then deinterlacing and keeping the full 1080 wide data might be worthwhile, but a lot of 1080i video just isn't sharp enough to justify doing so.
So those are what I think are the easiest ways to deal with the interlaced video you have shot. Compressor is not expensive. Making presets in Compressor and using them in FCPX is now quite easy. When shrinking video down to smaller sizes, appropriate edge sharpening and careful compression choice can make a massive difference. So if you do export down from 1080 to something smaller, Compressor is absolutely brilliant and well worth getting.
Next time shoot progressive - make you life easier! :-)
PS - the above applies to FCPX, sorry, I didn't notice but you are not using FCPX and this is an FCPX only discussion.
Cheers - c
