Step 1) Get a burner and large brewpot (around 10 gal) to do full-wort boils. Keep doing extract batches so you can get the other parts of your process nailed.
Step 2) to do partial mashes, it can be as simple as a10t2 says - get a grain bag. You can put your grains inside, mash in your original brewpot (I'm assuming 2.5-3 gal). then when you want to sparge, you can simply use it like a giant tea bag and dunk it in 3 or so gallons of 168F water in your large brew pot. You can also go as far as making a mini mash/lauter tun with a cooler and false bottom, but you should probably just make a bigger one and go all grain
Step 3) Get yourself a big (at least 10 gal) cooler, build a false bottom/manifold/ss braid and build a real mash tun. By this time, you have everything you'll need for simple batch sparging - your large kettle, burner, small kettle (for heating strike water and sparge water) and your mas tun.