Need Help from the Minecraft/Computer Gurus here!

You may have seen some of my other posts lately. Been getting into the whole Minecraft thing and recently started fiddling with texture packs and snapshots. But now I've moved on to allocating more memory and adding mods (like Optifine, which I hear is good) in order to make Minecraft run better. Thing is, I'm hopeless with computers. So I need your help (again). My computer is kinda slow, and I only have 1.86 GB usable RAM (which I hear isn't much). Anyway I could modify this and add more memory for Minecraft to use? Also, a I've tried getting a Mod Manager for downloading mods, but could someone go through a step by step intruction set for what to do? I read comments that say "download magic launcher" but I don't know where to put the file I download and what to do with it when I have it.

Please help! I love you all, you have always been so helpful in the past! Thanks ahead of time!