If you're a beginner, I'd suggest getting Lightroom first, it's a great tool for organizing photos, developing raws, and exporting your work.
Lightroom allows for more precise manipulation of light and colors than Photoshop, without touching the original picture, and as you can copy/paste any combination of settings from one picture to another, it's also quicker when working on a photoshoot, for which you generally want homogenous photos.
Once this work on light and colors is done, you may want to open it in Photoshop to correct some things (removing blemishes, cleaning the bakground...) or because there is some light/color effect you only want to apply to a part of the picture.
One of the best pieces of advice I can give you for Photoshop is : ALWAYS use layers while working, and always keep an unaltered background layer until you're finished. Learn how to use layers, as well as layer masks. If you do several edits on your picture, use as many different layers.
For example, I'm currently going through a series of pseudo-studio shoots I've done in July. We did not have proper lighting studio gear or studio backgrounds, so there's a lot to correct. For any picture, I generally have these layers :
* a background layer with the unedited picture
* above it, a layer to correct the background (wrinkles on the white sheet we used as background, dirty floor) ; I mostly use the clone stamp for this.
* above it, a layer to smooth skin (everyone has a different technique for this, mine is to make it extremely smooth, and then put the layer at 50-60% transparency to recover the detail of natural skin under it).
* above it, a "selective color" adjustment layer that makes the whites whiter (in order to even the background even more).
Each layer above the first has a layer mask associated to it, so that the correction only applies to the parts of the pictures I want it to. It's especially handy with the background correction layer : I can do the clone stamp thing very roughly, and then fine-tune as much as I want on the layer mask with the brush and eraser tools.
Hope this helps, and sorry for the grammar