Get out and push!
Yes, I am serious. It works.
Go on an EVA, activate the jetpack, and fly against the prograde-facing side of the capsule. When you are nearly out of EVA fuel (right-click on the astronaut to check how much you have left), get back into the capsule and leave it again to refill. You can do this as often as you want.
It doesn't do much, but each time you bump against the capsule, you bump it back a bit reducing its velocity. With enough patience you can get the capsules trajectory sub-orbital. You only need to get the periapsis below 70km. Atmosphere friction will then do the rest and deorbit your craft. Then you just have to activate your parachute and recover the craft.
...you do have a parachute on the craft, don't you?
No? Well, then all you can do is:
Do a rescue mission.
In order to rescue a kerbonaut you will need to perform an orbital rendezvous. This is an advanced maneuver to pull off, and it is even more difficult when you haven't unlocked maneuver nodes yet. But learning it is very important because it is the prerequisite for docking which opens countless new opportunities.
Build a more powerful ship than the one you built before, especially with much more fuel on the upper stage. Then add a probe core, some batteries and solar panels so it can fly unmanned. When you play career and don't have the technology yet, you can also just stack two command pods on top of each other. It looks quite ugly, but it works. Before you launch the vehicle, go to the crew-tab in the vehicle building and make sure the pod for Jebediah is empty.
Launch the rescue vehicle a few minutes before Jeb passes over the KSP to make sure you get close to it.
During and after the launch try to get on an orbit which is almost the same as the one of the ship you want to meet. When you timed the launch right you will end up quite close to the target, but it is quite unlikely that you will get it perfectly right, especially on the first try. Likely you will end up a few degree in front or behind the target.
When the target is in front of you, go on a lower orbit. When it is behind you, go on a higher orbit. The vessel on the lower orbit will catch up to the one on the higher orbit. When you are within 5km of the target, you can boost directly into the direction to close the distance. When the ships are below 100m apart and have no relative speed towards each other, you can use an EVA to transfer Jeb to the rescue ship.