Sarah, here are some ideas, not knowing what else you do to build on them I'll just start at the beginning. Listen to the advice already mentioned, some good omega's etc., but the most important thing would be to first check your body fat, and know where you stand. Before trying to bulk up you may want to know what your actual goals are. But if you are already too "underweight" which you will find out if you get your body fat% checked, you will have to add more food before adding more exercise.
That said, do squats, lunges of all forms, and on your hands and knees do leg extensions that pulse back. Let me explain that one though. You are on your hands and knees and have to stabilize your spine, your full core, and do not allow yourself to twist, but keep your hips even, your head only slightly elevated but still looking at the ground (don't drop it or pull it up to stare at the wall, you want your spine in stabilized neutral). Now extend on leg back straight behind you, then curl your leg up till your foot is pointed at the ceiling. From here, pulse your foot up toward the ceiling using your BUTT NOT YOUR BACK. Your lower back shouldn't move. This is not a huge move, but nicely intense and feels great... at least your butt will later

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For abs, try some intense ab exercise like bycicles... extending out the legs at a 45 degree angle one at a time, pulling the knee in to meet your opposite elbow... I am sure you have seen this one. Just make sure you are using your stomach and obliques to do it, don't cheat and bring the elbow forward to meet the knee. Other than that... flex your abs all day

. You don't NEED equipment, you just have to be imaginative. Any more I'll talk later...
