By now, your baby's physically well developed. When lying on her back, she'll lift her head and shoulders as you reach to pull her up. If you place her on her stomach, she'll extend her arms and legs and arch her back. This is good exercise for strengthening her neck muscles, and it will help her develop the head control necessary for sitting up.
Now your baby can anticipate the direction you're pulling her, and her head will follow right along with the rest of her body. What's more, her spine is straightening, allowing her to sit unsupported, which frees her hands for exploring, and she can pivot to reach a desired object.
Once her back and neck muscles are strong enough to hold her upright and she's figured out where to put her legs so she won't topple over, it's just a matter of time until she moves on to proper crawling, standing, and walking. Until she can get into a sitting position without your help, you can prop her in the corner of your sofa or on your legs to steady her.