People just talk about that you need to magic find "things" but thats a really weird and unspecific answer.
If we talking about normal, you can kill infestor even with a level 15 melee char. Infestor and his group deal a lot of fire damage. So if you are struggling specifically against him, I would say that your fire res is quite low. An easy and strong solution is a 2 socketed helm with 2 ral runes. You can get them very quickly by running normal countess (Act 1 Blackmarsh). 2 ral runes will give you 60% fire res. There are ofc other options like buying a 3 socketed shield from Fara in Act 2 and socket it with rubys or diamonds.
The second problem might be that your life pool is way to low. If you put a lot of attribute points into energy and dexterity, you don't have enough life. The easiest solution for new players is just to have enough str and dex for your gear and the rest into vita.
Third solution (used in almost every speedrun of diablo 2). Just don't be a melee fighter. In general as a meele fighter you can't really kite (hit and run). You have to be strong defensively and offensively. That means that you need pretty good items. On the other way with a proper hit and run technique as a hammerdin, you can even clear cs with 0%/0%/0%/0% res quite easily. The only option of "abuse" you have is to do choke point hitting. Make sure that your character does get surround from 360°. Stand in next to a wall and reduce the angles to 180°. Stand in a corner and reduce it to 90°. Sometimes you can find a good spot where only 1 enemey can hit you.