First thing I would do is buy a good router.
What most of us in the network field own is a Mikrotik. they are far cheaper than all these junk routers you find at places like walmart bestbuy or other chain retail stores.
Mikrotiks RouterOS Operating system is light years ahead of everything else on the market. It's got very advanced options, but they have a sort of quick setup for the less inclined users and works just fine without having to do much more than pop in your basic wifi info.
Just for reference, my brothers $400 router with antenna sticking out of it like some sort of porcupine got 1/4 the range of my $60 Mikrotik router that has no external antenna. His and all other routers I've tested from the chain outlets all also require rebooting pretty much weekly where the Mikrotik simply never needs rebooted.
here's their AC version
https://www.amazon.com/dp/B01BMMK4HI/?tag=httpwwwtechsp-20
here's the one I have
https://www.amazon.com/gp/product/B00EQWUZM6/?tag=httpwwwtechsp-20 bought back in dec 2015
Now should you actually need more signal, you can adjust all the way up to 1000mW but you likely will never need to adjust that.
Do understand more mW isn't always a good thing, too high and you'll gain more noise in the signal causing speed and/or connection issues.
if you need more signal strength for some reason, then you could do one or two things. first getting a better wifi adapter for the machine you're using, typically if it came with your computer, they tend to skimp on those parts opting for the cheapest one that has the "keywords' you'll be looking for. other option is to add antenna to the router as a mod.