Ok, I'm fighting to avoid making puns about Hendrix songs. I saw him play, twice.
Wintellect gives good answers. PCBSD is probably a good introduction, giving you a chance to see if you like it with a lot of the work already done for you. It installs with a complete KDE desktop.
As he (Wintellect, not Hendrix) says, define customize. Linux ranges from the completely bloated "You, the users, are too dumb to have much power," (most of them these days), to those like LFS, Gentoo and Arch, that give you a fairly minimal installation with a lot of options for customization.
However, if you mean customize as in a typical Fedora or Ubuntu installation, changing many things through the desktop tools, such as choosing your own background and such, that is a function of the desktop or window manager, not the O/S. Therefore, customizing fluxbox on Linux or BSD would be the same.
As a beginner, the big difference would be the upgrade, rather than kernel config/build procedure. As phoenix has said elsewhere, it becomes less and less necessary to customize the kernel. Many of us still do, (out of habit?) but these days, the GENERIC kernel can be left alone if you're a beginner, without any mjor differences.
Oh, the default shell might throw you off. FreeBSD uses tcsh as its default, rather than bash, and an ordinary user gets sh. (Hrrm, I realize how long I've been away from the BSDs, I'm not even sure that I'm right about this, and I *think* PCBSD might put bash as its default.)
If PCBSD does use bash by default, then that will be similar save for writing shell scripts, as you would use either /bin/sh or /usr/local/bin/bash vs. the Linux /bin/bash.
One other thing that sometimes throws first time BSD users is that it does require a primary partition on your hard drive. Linux can use an extended partition.
I have a little page that I wrote as the question comes up often on Fedora forums. It's pretty Fedora centric, but does mention some things you might find surprising.
<@andre> i would be so much more efficient if i wasn't so stupid