Note: for the purposes of this answer, I have assumed no tech remains. If you do have tech surviving, then many of these steps will be easier.
There seem to me to be a number of steps that you must go through to rebuild a planet.
Water is the most basic human requirement and without it everyone dies very quickly. The first priority is to find a source of fresh water that people can drink. I do hope your apocalypse wasn't nuclear or this is going to be very hard.
Food is the next requirement: while you can survive longer without food than you can without water, you're quickly going to need some to be able to keep rebuilding. Look for old stashes, if there are any: anything in a tin will have kept. As soon as possible you want to start getting some basic farming going. Fishing will also be a good option, especially if you're near the sea.
Tools are also fairly essential. To open tins and kill things to eat you're going to need some, even if it's just a sharp bit of rock. Start your search for tools quickly, preferably at the same time that someone else is out looking for food.
Start building. You need somewhere to survive. If you're already in a cave you're in a fairly good way, but you might want to consider partially blocking entrances to keep warmth in. The more people in one shelter, the better, as it will be warmer inside. However, caves can't sustain you forever and you're going to need to start building other shelter soon, but that requires wood.
4. Start Growing
Trees, plants, food, start growing it. You'll need them all to survive, and many plants we think of as useless do actually have very useful functions in a survival situation: nettles can be used as string for bows and you can make tea from them (though that may be a while off yet). Trees will be essential for building better shelters in the future.
5. Start Exploring
Now that your camp is reasonably well set up and sustaining itself, get a team out every day, exploring the surrounding landscape. They should look for:
- Food sources
- Water sources
- Better shelters
- Raw materials
- Other people
You've got a good setup for your little group, so you can now try to make contact with other groups of survivors. There might be one 2 miles over the hill; your explorers should find them. Any lost souls you find, you can take in: you should have some surplus food and water, and it strengthens your group.
6. Start Building
This should now be several years down the line, where you have food, water and materials stored up. You can now start building: homes for people, storerooms so your cave and homes aren't so cramped. As you get new materials in, invest them in building to last - you want what you build to survive as long as possible so you don't have to expend materials repairing it.
7. Start Innovating
This is even more years down the line. You should have a permanent camp set up and teams going out every day to get food, water and materials. Set up another team or few at camp to start making things that might be useful for you:
- New tools
- New storage methods
- Methods of communication
Obviously we're not talking about radios and all that yet (unless you happen to have the materials and engineers - but they're still pointless unless someone else has one), but any way of getting your voices heard further and further away. Perhaps a primitive voice amplifier? Go up to the top of the nearest hill and shout. See if you get any responses.
8. Join Forces
Your group is now strong. You should be looking out for other groups and other lone survivors so you can join up with them. You want to get more and more people together, because then you can start rebuilding civilization.
I should also note that in most of these steps, you're going to need more and more advanced tools than the last. You might want to set up a team of engineers at base to make you new tools.