the side you put sheets on

350 is good if you're working on wires, 400 for connecting to boards (only touch the metal and touch it like it's tissue paper, or just touch the leads since they won't take damage), 430 for larger parts, 450 and a heat gun only if you know what you're doing (the line between heating the board enough to attach the part and watching every other part fall off is most easily found by watching all the parts fall off). no-clean flux is a good choice for diy projects, but if something needs to last more than 20 years the flux will cause shorts.

you're probably going to want to add flux to whatever surface you're trying to connect, hover your iron over the spot while adding solder to your iron, then force the ball of solder over the connection. you might need to do this multiple times as wire can absorb a lot of solder. have something hold the wires in place as far from where you're heating if you can, otherwise try to twist the exposed metal together, or twist a single strand of wire around them to keep them connected. you also can't solder aluminum (at your skill level) so holding it together with tweesers could be an option.

can you describe the parts you're going to connect? soldering might not be the only option.