Ok, you're well on the way to being able to do signaling. As I said above, you're going to need some type of signal driver board. The SE8C is Digitrax's board for driving signal heads. It supports up to 32 signal heads. So you'll need to count how many heads you expect to need or want. Typically, you'll need 4 signal heads per turnout. You may also want a couple signal heads between adjacent occupancy blocks if there's no turnout between them. Do some studying of railroad signaling so you understand the basics of where they're used and why. You may want them to just "look pretty", which is fine, or you may want them to act more realistically. Getting them to act realistically will require more sophisticated programming which JMRI should be able to do. I use a software program called TrainController rather than JMRI, so I can't help you with JMRI specifics.
I have three SE8Cs for my layout, primarily because of the number of switch machines I needed to drive. I have a total of 44 signal heads on my layout. To save money, don't put signal heads where you can't see them, of course. If you end up getting higher detail signal masts, you may choose to have them be unlit where they're pointing in a direction where their lights can't be seen. If you go with the SE8C, you'll also need wire/cable to connect the SE8C to your signal heads. Digitrax makes the SDCK Signal Driver Cable Kit for this purpose. This kit is very overpriced, but the concept is great. You can buy equivalent ribbon cable and connectors online through an electronics supply company for much less cost. If you go this route, let me know and I can look up the part numbers and company from which I ordered my cable kit parts.
Working signals look great and add a lot of realism to your layout and are really gratifying to get working. Let me know if you have more questions.