There are hundreds of ways to start a fire in the field when needed. I have done many of them, but I am going to describe two basic ones, that have always worked for me. (Hint-it is not rubbing sticks together).
The basics of a fire, are fuel source (tinder/wood), oxygen, and ignition source.
Oxygen- luckily is all around us, however one must keep in mind that at higher altitudes there is less in the air, thus getting a fire started is much more challenging.
Fuel source- the material must have a low moisture content for it to ignite well. If have wood in timber that has been rained on, we are looking at wood, that when you pull on it, it makes a "crack noise", like a fire crackling, this will be dry enough for the fire. Gather plenty of wood, that is like this.
Ignition source- Of the thousand way, I am only going to recommend and talk about 2 that never let me down. Bic lighter- they are cheap and easy to use, I put a little duct tape around it and have two in my pack. Ferrocerium rod (Fero Rod)- a metal alloy pen shaped rod, that when struck produces sparks (Note: do not go cheap wally world one, get a Überleben Zünden Fire Starter, Light my fire, or Eotac one.)
Now the secret to getting a fire going quickly, is the tinder bundle. A tinder bundle is small dry material, that is bunched up, so when a flame/spark hits it, it will catch and burn. Picture a birds nest under your outdoor porch. It is dry, small twigs that has room for oxygen to flow through. You can also make a tinder bundle, from wood shavings off dry branches, as Mori Kochanci describes in his book. Whatever you have, I want you to place them, in a row. The small tinder bundle, then small twigs, then little big bigger twigs (about thick as pinky finger, in half), then wood size of pinky finger, then up from there. Now I am going to share another little trick that makes it even more efficient and I have these little items in my pack always too. One is cotton balls, that have been rubbed with petrolum Jelly, the other is few small birthday trick candles. Both are cheap and easy to use, the cotton balls catch spark quick, the birthday trick candles resist high wind good, and once fire started put out for later use.
Okay, we have our fuel source laid out, now take the PJ cotton ball, and fluff it up a bit, use two if want but one will work fine. Now that it is fluffed up, place it in middle of the tinder bundle. Take your Fero rod, notice that it is color black, go ahead and scrape a lot of the black off, so you see silver. The black is protective coating, the silver is what we are after. Now place the rod down by the PJ cotton ball, using a scraper/spine of carbon steel knife/spine of saw on swiss army knife, lay it on the rod. Now, do not push the scraper into the PJ cotton ball, as will push it away, instead, pull the rod away from the scraper, as you push straight down with the scraper. The sparks will be thrown into the PJ cotton ball (with a big lighter, do same with sparks or flame).
It will light, now roll it into the dry tinder bundle with stick/knife, if it just smolders, carefully pick it up above your mouth about 8" away, and lightly blow air into it (this way any spit/moisture, drops instead of going into dry bundle). Once it catches flame, set it down and slowly start adding the half pinky size sticks (popsicle sticks work great too). Do not smother it, we have to nurture it, and get solid embers going, not done yet. As the sticks catch fire, add few more. Then add one stick of pinky size stick, then keep adding the half pinky ones. Once, the pinky size catches flame, add few more. It is at this stage, you should have a small fire, that you can add bigger and bigger sticks.
If you have a charcoal grill at home, you can lay some foil down and practice this anytime, this is how I taught my kids to build fire. I recommend keeping the PJ cotton balls in little duct tape ball, as the duct tape torn in strips can be used as kindling too.
One other tip, I have worn glasses all my life and found this out early on. If you have sun, you can use your glasses to focus the sun's rays down to very small thermal point, that will catch kindling too.
Make up few PJ cotton balls, keep a few in your pack with lighter/ferro rod and you will always have means to make a basic fire. So you can stay warm, boil water.