[I cant play this modpack for more than 5 minutes without crashing]

Im not very experienced with downloads and file things so if anyone could give me a thorough explanation of how to get my modpacks not to constantly crash. I used to have believe it or not a mac book air and my modpacks could run on that for a long time and they would not crash, so it must be something with my settings. I would really appreciate some help

[Crashing]

If you could can you just explain a few steps for me to do that? im just a bit new to installing mods and all the computer setting things.

[Crashing]

Everytime i try to make a new world on tekkit legends, it alllows me to play for maybe like 2 minutes, then it just crashes out of nowhere. I dont know any reasons for this to be happening.