A few fundamentals...the drain lines must slope 1/4" per foot. You need to find out where your sewer line is. If the slope from that area to the existing line is not the right pitch, you can still do this, but you need to add a pump and a basin. Gravity is always better if you can do it. The toilet needs a minimum of 3" line, a shower 2", and normally, anything under the slab needs to be at least 2" - but a vanity only needs 1.5". You don't want to make many turns in the drain line, and may want to join the sewer somewhere outside rather than tearing up the slab in the house to attach.
A garage is often on a sloped floor, so depending on what you want, there are ways to level that up that aren't a big deal, but you may not need to do it, just be aware of it - shims on things may be required.