Read the overclock.net guide. It will tell you in detail what to do. You can't overclock if you don't know what you're doing exactly. It's a bit of a science and sadly isn't as easy as just changing a single number.
Overclocking isn't a good way to gain a large amount of FPS also. It won't turn a 2008 era CPU into a new one. Same goes for graphics card overclocks, they are only for squeezing a bit more performance out of a card. Most people here would tell you that that system is too old and under powered to play any kinds of games on in terms of CPU and GPU. One thing you may want to do is get a better graphics card. The performance you get from the R7 250 is comparable with a new PC with no graphics card at all. That would give you more FPS immediately although then you'd have your CPU bottlenecking.