Pro tip: If you know how to do it safely, pre-drill any holes they will need to run their cable and pull a string from wherever you want the cable modem to be to the hole where they will bring in their cable (usually right next to the electric meter, since by law they are supposed to use a grounding block connected to the house ground which is usually only available by the electric meter). Of course that puts it on you to avoid hitting anything you shouldn't, such as electric wires, or water, sewer, or gas pipes, etc. But I guarantee you that you will be a lot more careful with your house than those guys will. As ReggieA said, YOU tell THEM what you want them to do, and watch them like a hawk, because the minute you don't they will be drilling holes where you don't want them to or otherwise damaging your property. They have no business touching any of your existing wiring or equipment unless you want them to, and I won't even let one of those guys take an electric drill off their truck, much less into my house (unless they are willing to let ME drill whatever hole they think they need).
Also you may want to have a tube of silicone caulk handy (the real thing, not that gawdawful "siliconized" crap that should be thrown in a dumpster) so that you can seal the hole(s) going into your house after they run their wire. Many installers couldn't care less if they leave a hole unsealed (or if they use lousy caulk that cracks or peels away after a short time) and water gets in and rots your wood and invites carpenter ants - it's not their house and they don't have to deal with those problems. You do.
Yes there are good installers that do care about the quality of their work, but don't count on getting one!