There are organic herbicide options that will kill grass but not flowers, including white vinegar or a dish soap solution. Commercial selective herbicides can also target grass over other plants, but read the instructions carefully to avoid damaging your garden. Herbicides that kill grass but not flowers usually include ingredients like clethodim and sethoxydim. Otherwise, your best bet will be to weed by hand.

Yellow leaves and stems on your sprouts indicate they’re not getting enough light. Keep them near the appropriate light source and give them as much as they need (they require longer exposure to fluorescence than they do to natural light sources).

Several tools exist that can help remove unwanted grass. The one you choose depends greatly on how much you need to remove and how hard you'd like to work. A sod or turf cutter is the fastest method while using a shovel is labor-intensive and much slower. You may also need a garden rake, tiller, and wheelbarrow. 

No, definitely not. Pulling weeds by hand is the single most effective way to remove them because you're eliminating the entire plant—roots and all—so it won’t have a chance to grow or spread. Setting time aside a few times per week to pull weeds can keep them at bay. If the number of weeds gets overwhelming, then you can resort to other methods.