As others stated in the comments, you must make the application, not your sponsor.
The money you have available to you is of secondary importance. What is absolutely critical is that you can convince the entry clearance officer that you will return home after your trip. They start with the presumption that you won't. You must include as much evidence as possible of your employment, since this can counter the ECO's assumption that you will overstay.
Therefore, you should include a letter from your employer detailing your position, salary, length of service etc. and stating that your trip is authorized.
You should include as many bank statements as you can, showing that you regularly receive your salary, and that you are building up savings. There is an excellent answer here explaining how the ECO will interpret your bank statements. Don't be tempted to have friends or family transfer you extra funds, since this will look like "funds parking", and could lead the ECO to believe you are lying.
Do not be tempted to take too long a trip. People with jobs don't usually take long holidays (longer than two weeks). A long trip will cause the ECO to doubt that you have a stable job. Do not fall into the trap of staying longer than you say you will. If your visa is granted, you may receive a visa for a longer time than you asked for. Regardless, you should return home on the day that you said you would. Doing otherwise could lead to future visa applications being refused.
As the government guidance states you must also supply an itinerary, where you will stay, and how much you think it will cost.
Even if you do everything right, you may still be unable to overcome the presumption that you will overstay. In this case, it will be fruitless to reapply unless you have extremely convincing new evidence to the contrary, or your circumstances change (for example, you get a promotion or a raise). You could consider meeting your girlfriend's family somewhere outside the UK, where there will be less of a presumption that you are trying to come to live with her.