Select your footage in the Project panel and then right click or go the File>Interpret Footage>Main and check the frame rate. Set it to what ever you want for your project unless you have sync sound. If you have sync sound then you should usually use the native frame rate of the footage. You should not change the standard 23.976 to 24 unless the majority of your project was shot at 24. Sometimes footage shot on a phone or other consumer device is slightly off. You may see 24.1 or 23.2. If that is the case fixing the interpretation to a standard frame rate will not change the sound quality enough to worry about so fix it. Pick the nearest standard frame rate that corresponds to your project requirements. Once you have the frame rate interpretation checked or set to the correct value it's time to trim set the in and out points of the clip you are going to rotoscope.
To set the in and out points of the footage double click the footage in the Project Panel to open the footage in the Footage Panel. Move the CTI (current time indicator) to the desired in point. It is often a good idea leave yourself a few frames for editing the final shot. The extra frames are often called handles. Now set the in point by using the keyboard shortcut Alt/Option + [. Now move the CTI to the out point plus any desired handles and set the out point by pressing Alt/Option + ].
Now select the clip in the Project panel and either right click or go to File>New Comp From Selection to create a new comp. You can also just drag the selected footage to the Create A New Comp icon at the bottom of the project Panel. It's the third from the left.
Now Select the Footage layer in the Timeline and then select the RotoBrush tool in the toolbar and either right click on the layer name and select Open Layer or double click to Open the Layer Panel. At the bottom of the layer panel observe the Magnification Ratio, far left and set that to 100%. Make sure the CTI is on the In Point of the footage. Paint your first mask and observe the Rotobrush Refine Edge Span right under the CTI. Drag the right edge of this span to the right until it matches the out point. Now refine the first mask until you get what you want. To propagate the mask it's usually easier to press Ctrl/Cmnd plus right arrow to move forward one frame. Observe what is happening to the mask and move down the timeline as quickly as is practical making adjustments to the mask by adding or subtracting until you get what you want. Holding down the Ctrl/Cmnd key while dragging will change the size of the mask tool. Holding down the Alt/Option key will change paint mask to subtract mask. When you have reached the Out Point go back to the in point and check the mask by carefully observing all edges. If you have to add new corrections the you will have to propagate the corrections all the way to the end of the shot. Rotobrush only propagates to the right. Learn how to use the various Toggle Alpha Options (bottom left icons) to preview your matte as you make corrections.
Once the mask is where you want it select the Refine mask tool, move to the In point and start again to refine the edges. Again, propagation only happens moving to the right and any changes you make after checking the mask will require you to propagate or advance one frame at a time until you reach the end of the clip.
NOTE: Some scenes may work and propagate correctly by using the space bar but in CC 2015 using the space bar to propagate the roto is usually a frustrating experience.
Once the mask is complete and the refine edge is complete you should check the Freeze option at the far right of the Footage Panel. This will lock the roto so you don't have to do the entire thing again the next time you open the project.
IF YOUR SHOT is more than a few seconds long you will be way better off to render a Lossless With Alpha Digital Intermediate (DI) of your clip before proceeding with the rest of your compositing work. The time spend rendering the DI will be saved many times over in complex projects that are more than a few seconds long.
I hope this helps. Rotobrush takes some practice and expertise to use well, is not a single click and let it go solution, and is a huge drain on system resources. Rotobrush also will not work on all footage. Noise, motion blur, exposure, rolling shutter artifacts, compression artifacts, focus, and the suitability of the subject and the background have more to do with the success you are going to have using the tool than technique ever will. It is not a tool that will fix every shot. In my experience it will actually only reliably fix those shots that were shot with Rotobrush in mind.