Hello Lee welcome here to the forum.
There are a number of visa options available to you, however you'll need to be clear about your strategy.
If you want to have your g/f to join you in UK to continue development of your relationship your best route would be with a General Visit visa.
The UKBA page specifically for UK Visit Visa's can be found here and will help you to choose which category is the most suitable for you. It also explains what you are allowed to do while you are in the UK as a visitor.
It's well worth taking a look and checking out all of the links provided.
Anyone coming to the UK as a visitor, will usually be granted a visa with a validity for up to 6 months.
The cost of a visit visa is very low at £78
On the other hand if you are both sure of your relationship and you both want to settle here in the UK then the standard route will be either an application for Fiance(e) / Proposed Civil Partner for marriage in UK, or alternatively, if you're already hitched, then it'll be an application for a Partner/Spouse/civil partner visa.
In principle both of these routes lead to settlement and have much the same eligibilty requirements.
It has to be said though that of late there appears to have a few Fiance(e) visa refusals based on the ECO not being convinced on intention to get married.
There are forum guides for both Fiance(e) and Spouse Visa that might help.
Take a look here for Fiance(e)
and here for Spouse
The cost of these visa's are rather expensive at £826
Good Luck with your quest and in whichever route you feel is best for you.
