A good basic explanation: https://www.makerspaces.com/how-to-solder/

Soldering Tips

There are different kinds of soldering tips that can be useful for different projects:

The beveled tip is the most common/"general" soldering tip and can be used for most soldering jobs, but exchanging soldering tips may make certain soldering projects easier. You will most likely only ever need to use a beveled or conical tip.

Flux

Flux, sometimes called rosin, is a gel-like or liquid material that can be applied to joints to make soldering easier. The solder will be more likely to attach to where the flux is applied, and so is good to use if you find the solder is sticking to your tip and not transferring to the joint properly.

Kinds of Solder

There are two main kinds of solder: lead-based and lead-free. Lead-based solder is the most commonly used, but due to the negative health effects of lead, the industry is moving more towards lead-free solder. Lead-free solder is a bit more difficult to work with, but not by much. Along with these two solders, some solders include a rosin (flux) core that improves connections. There are many different kinds of solder in the two main categories that combine different materials; here is a good website that tells you some of the common ones and their purposes: https://resources.pcb.cadence.com/blog/what-are-the-different-types-of-solder-2.

Tips:

• If you're finding that the solder is not sticking to your tip, clean it off with a sponge (brass or conventional) and try again. Alternatively, apply a bit of flux to your tip.
• Keep the tip near the joint for as little time as possible - applying too much heat to components can damage them.
• Remember that you can desolder! It can be an annoying process but don't panic if you accidentally connect pieces you didn't want to.