problems with soldering

[t-maxxracer32]

hey well idk where to put this but i suck at soldering lol. idk if its the solder i have ro what but im using this really old weller soldering iron and it heats it up fine but the solder just wont stick!! im using some offbrand solder i think (has no label) lmk what kinda solder i can use and if i can buy it at lowes or something.

thanks!