I have around a 20.5 bmi, and I'm pretty lean. However, since I don't have any fat, my ribs show prominently. Is this bad? And how do I fix this? (I don't want to gain fat, I'm looking for a workout I can do or something)
BMI is stupid. There are just too many body types to really make it worthwhile, and unless you fit the average body type that it's made for, it's a completely worthless number to you.
Strength training, specifically workouts that work your core muscles. Also, don't do 1000 crunches, this will get you nowhere. You want to do the work against enough resistance that you can only complete 8 - 10 reps continuously and do 3 sets of it.
The muscles that cover your ribcage are your obliques, therefor you want to do exercises that stimulate this muscle. You ever seen a male model and on the outside of his abs he has that kinda wavy muscle mass? Those are your obliques. Research on the web exercises that you want to do.
Personally I suggest that you do an all inclusive weight training program based around compound lifts (involving two or more joint movements) like squats, deadlifts, or overhead press. Bench press works too, but try to do a single armed bench press with barbells, this will better activate your obliques to help stabilize your body as you lay on the bench.
Eat lots of plain ham sandwiches or anything else with tons of protein and not a lot of carbs/fat, and work out with heavy weights every single day for an hour.
If you absolutely don't want to gain any fat, only eat every other day while doing this. If you have to break eat mustard or salt or only a few slices of lean meat like ham slices. This is what I did, and people always say it's really unhealthy and honestly it really sounds that way, but I never suffered under it, so I don't know. Maybe only us young dudes can get away with it? I've never done it for over 2 months though.
By heavy weights I mean stuff you can only do about 8 reps of for 3 sets. Even if you can't finish the last set and you can only do like 8 reps 8 reps 4 reps of a workout at that weight level, that's good too. You don't want endurance you want power if you're going for a less lean more massive look.
Also I'm a quarter polish and my ribs show no matter what, a lot of polish people have a big core body and that's just the way it is, so if you're a burly mofo but a few ribs show, you're still golden (I know from experience).