If this is a one-piece fiberglass tub and surround (molded together) then you're in for a big job, as the tub would have been correctly set in a mortar bed.

If you're saying you have a one-piece or multi-piece SURROUND only, then it can easily be removed. Expect to take the wall finish along with it (dry wall, whatever) and/or that you will be damaging it beyond redemption and be replacing that as well. (You'd likely be doing that anyway to properly prepare to tile anyway.)

Taking it out in one piece not only difficult but silly to go through the effort. Cut it up, much easier to tackle de-laminating it from the wall and removing it anyway - just don't cut your studs or plumbing behind the wall, or - heaven forbid should there be any electrical behind the wall.

Find your studs and mark their location above this unit before you begin.

If you're referring to something installed in a manufactured/mobile home (where the one-piece tub/surrounds are common) - then you'll need to take a completely different approach, as the tubs are installed before they make the walls so you'll be in for a real mess.

Have you considered going to a marine supply store for a fiberglass paint filler?

We repaired fiberglass boats in the Navy that had severe scratches?