I am completely new to Linux. Ready to get as much advice and tips as anyone can offer. First, which version should I install? Are the free online versions safe and functional? I am running windows 8.1 on a laptop. Would prefer to install, if possible, any type of Linux on USB or disk. If not possible, use a partition. Everyone has to start somewhere, right?

All of these questions could be answered using google.com.
Whichever one you want. Maybe take a look at distrowatch.com to find a distro that fits your needs.
Yes, at least the popular ones that are being actively developed.
Linux can be installed on a disk. Use that google.com thingy to find distros/instructions for installing to a flash drive if that's what you meant by USB.

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One easy way to start is to order a ready-to-use CD/DVD/USB of a live version from one of the vendors listed on Distrowatch. I used OSDisc.com when I was starting a few years ago, might work for you.

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Try Mint first -- it's designed for newcomers and has all the CODECs you might need plus a nice "welcome" screen with tips & help.
http://linuxmint.com/

GNU/Linux is fine installing to a USB drive or even a memory stick.

You may have issues with some distributions in respect of Secure Boot (Ubuntu, Fedora & openSUSE will install & boot with this enabled) and UEFI booting (most of the main distributions now support this, but some don't).

The linux kernel has a number of different operating systems on top of it like Debian, Ubuntu, Arch Linux etc.
Each OS has different aims and aspirations, so to find the OS for your needs you'll need to read up at least a little about each. It can't be done quickly in a single thread.

The operating systems then have desktop environments on top of them like gnome, kde, xfce etc. Their aims can be to be slimline or simple or elegant or full-fat and user-friendly.
You'll need to decide which one you want to start off with.

The Linux OSs don't have anti-virus software. This is because they don't need it.
The stable releases run fast and are they are reliable.
If you watch some stuff on Linux and Gnu project on youtube, you'll find out more about what you want.

Good point.
If you use a live usb or cd, then you can install different OSs with different desktops. When you run them, you can see which ones are compatible with your hardware.
Then you can play with one version without any problems running it 'out of the box'.