Alt-Tab usually brings whatever is behind the front window back to the front.
Often highlight text, right-click, copy, go to terminal and Shift-Insert will paste. Also you can do the same with hightlight, Ctrl-C and Shift-Insert in the target window.
In some terminals (not xterminal or rxvt, to name just a couple) you can also access a right-click menu, or one at the top of the window, where you can choose to paste from the clipboard.
Those are how things usually work. There are desktop environments where they don't. But you being a new user, I have doubts you're using FVWM or TWM as a desktop. Most likely it's Gnome of KDE, and those commands work fine in both.
Highlighting will copy text to a different clipboard, and then hitting the middle mouse button will paste it. Sometimes based on setup, the middle mousebutton may not work. Or you may be using a mouse that doesn't have one. In that case clicking the left & right buttons at the same time does the same as a middle button click. This particular method of copy/paste should always work in any enviroment that uses a mouse.