I'll give it a shot. I ain't no expert or anything so if this is screwed up, feel free to correct me.

First, before doing anything ... download all your files from wherever they're currently being hosted and back them up. Once you change your ISP info your old server may not work anymore and you won't be able to access your old links. (don't have a clue why but that's what happened to me and I was lucky I backed everything up first cause as soon as my ISP changed the info to '1&1', I didn't have access through my old web hosting service anymore and all my old links stopped working).

First thing you gotta have is your own domain name. If you've got one already, great. If not, you can buy one through 1&1 or whoever you want. Either way... you gotta have one cause it just makes life a whole lot simpler. Mine (schmange.com) costs something like $30 a year through a service called CA-Registrar.
Once you have your own domain name you need to contact 1&1 for the basic DNS info that your registrar needs to get everything working. Just ask em...they'll know what you mean.

Send your DNS info to your registrar and in about two days 1&1 will send you a message something like this....
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We are pleased to inform you that your registrar has made the required
name server entries and your Internet site hosted on 1&1 Internet Inc.
servers is now available through the domain schmange.com.

If you haven't already done so, you can now activate the e-mail accounts
for schmange.com: please go to menu items "E-mail > Activate External
Domain Names" in your Control Panel where you can find an overview of all
domains for which the 1&1 mail servers aren't activated yet.

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You'll have to get back to me and lemme know if I'm heading in the right direction here cause I have no idea how familiar you are with FTPing files etc..
and the process is a little different on a windoze machine.