One of these days I'm going to write up a tutorial specifically on node editing. You can do all kinds of things with the Node tool. A lot of people like to avoid node editing, if at all possible. But it doesn't bother me that much. Anyway, for now....
If you're in a hurry, skip down to the paragraph starting with an asterisk *. That's the direct answer to your question. All the rest is plenty helpful, but maybe you need more time to digest it.
You can move nodes with the Node tool. So you could move the end node to wherever you want it. If it changes the curve, you can change it back with the node handles. Click on a node to select it . Notice those very thin, straight, blue lines with a tiny circle on the end? You can grab that handle by the tiny circle, and move the handles around, which is how you can adjust the curve. If you don't see any handles, it means the path segment on each side of the path are straight
Any time you see a handle, it means that there's a curve in the path, on whichever side of the node you see the handle. It could be such a gentle curve, that it looks straight. But usually that's a clue, if you see a handle but you really wanted that segment to be straight. You can get rid of a handle and make that side perfectly straight, by holding the Ctrl key while you click on the circle. Or if you have a straight segment that you want to curve, you can essentially drag a handle right out of the node. Put the mouse over the node (it turns red when you're in the right place), and this time, hold the Shift key, then click-drag. You'll see the handle come right out of the node.
There are a few kinds of nodes, but mostly you'll be working with either corner nodes (diamond shape) which usually make sharp corners, or smooth nodes (square shape) which make smooth curves. For smooth nodes there are almost always 2 handles, and they always move together (when you drag one, the other one moves too). The handles on corner nodes move one at a time.
You can delete nodes by clicking on it to select it. Then either Edit > Delete or Delete key. Or you can add more nodes. Position the mouse over the path. When you're in the right place, the mouse pointer will change to the finger pointer. Then double-click. You can also add/remove nodes from the control bar (the bar just able the horizontal ruler, usually. It has different buttons and options and controls, depending on which tool you're using.
Also on the Node tool control bar, there are buttons for breaking a path and joining paths together. For the image you showed us, you probably don't need to do much breaking and putting back together. But you can only apply a fill color to a closed path. Typically newbies will draw whatever they're working on, with a series of open paths. Then they have to learn how to either join them together, or draw them all as one path to begin with.
*Anyway, what I usually do to make a path end where I want, if the path is a little too long, is add a new node at the place where I want it to end. Then delete however many nodes are extending beyond the new node. That new node absolutely prevents the curve from changing when the nodes on the other side are deleted.
Probably the reason I keep putting off writing a tutorial on node editing, is because it takes so darn many words to explain. I've never made a video tutorial before. But I have recently learned how to make videos. So maybe it will be my first video tutorial, haha.
Anyway, there's even more than what I've mentioned to node editing. A lot of different customizations and shortcuts. You can learn more about it in the manual (Help menu > Inkscape manual). Although it's not particularly well-written for newbies, it still does contain most everything you need to know.
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Paths.html (We're working on a new manual that will be written specifically for newbies, but it won't be ready for some months yet.)
Anyway, that much info should give you a good start. But feel free to ask questions, if there's something particular that you can't figure out.
Edit
Then there's the whole subject of aligning things - paths, nodes, objects. I'll just mention guides, grids and snapping, which you can look up in the same manual as above (different chapters). You can also place nodes on precise x,y coordinates, and you can find the X and Y fields on the control bar, where you can enter the values. Inkscape is well suited for drawing with geometric precision, in my opinion, although some people have complaints about how Inkscape does so. But that's yet another different subject.
Oh, I forgot, I meant to mention this info about using Inkscape on Macs. Since you have it installed, you've probably already addressed most of the concerns, but just in case:
https://inkscape.org/en/learn/faq/#Mac_OS_X_specific_issues
