I've done hatch struts on other vehicles before and it's easy (and I'm not all that handy). But I see reports around the net where Celica owners have had a few issues removing the strut end from the interior.
You can get aftermarket struts at autoparts stores. I'd check with one or two and ask the counterperson if they can take a look to see if replacement is going to be a simple unbolt process or not.
Usually you unbolt one end of the strut and the other end usually snaps out. Reverse to install. It's best to install new struts in pairs, but they are usually sold in pairs anyway.