Minecraft Mod Pack Server Questions

Ok well recently i have become REALLY bored with every minecraft modpack i've been playing due to lag, raiding, and not enough space. The question i am ultimately asking is how would i be able have a good private server (most likely voltz). I have access to my moms old PC its worth almost nothing but could possibly help. and im on a budget of $0.00 i am currently using a laptop with improving performance i have tried home hosting a voltz server/tekkit server/(well u get the point). I have allways run into the same problem... lag. on a yogscraft pack server i've tried home hosting i almost cant even move. if there is a way i can unlag the server id like to hear it. I will be asking my mom if i can use her PC for that. remember i am a 13 year old so no jobs available. all of my income is going to saving for the Xbox 1 from there i may just get a hosted server. Well thank you all in advance for any suggestions