UKVI has re-vamped and rationalised the whole Visit Visa process.

Their latest publication/guidance can be found at this link:-

Visit guidance

Some bedtime reading for you. It's not really complicated but if you have questions just fire them off. Plenty of good folks here to help.
Personally I'd hold fire on using an agency until you've understood the requirements and weighed up the risks.

A decent registered Immigration Advisor will certainly cost you and the visit visa is low cost and speedily decided.
If you really don't have the time or motivation then by all means engage a OISC registered UK advisor. But do be aware that they will be asking you to supply documents and written responses to immigration questions anyway.

The major complication is the need by the applicant to prove a non-immigrant intent.
Means that applicants wishing to come to the UK under a tourist visit must clearly demonstrate that they do not intend to remain in the UK on a permanent basis.
Immigration law places this burden of proof firmly on the applicant.
In order to grant a temporary visa, the ECO must be satisfied that the applicant has strong social, economic and family ties abroad that would compel them to leave the UK at the end of the temporary stay.

Unfortunately, in most cases it becomes difficult to overcome this presumption.

The key issues then for your g/f to seriously consider are:-
- Valid reasons to return home that outweigh reasons to stay
- Financials to cover visit duration.

In addition to demonstrating that she knows you and that you have some form of relationship/connection, she'll need clear reasons to return home with strong/complelling ties to her home country.
Such ties can differ from country to country, city to city and individual to individual.
Some examples of ties can be a job, a house, social and family relationships, a bank account, possessions etc

The ECO will be looking to see if the aspects of your g/f's life that bind her to her home country outweigh those aspects that may cause her to be unwilling to return to her home country, or to attempt to change or extend her visa status, so don't go overboard on the romantic issues.

I'd suggest including a letter from your parents supporting the application. Just a simple letter saying how much they're looking forward to the visit of your g/f etc

If you're intending to be a sponsor you should provide an invitation/supporting letter to confirm the support you'll be providing in terms of financial assistance and/or accommodation.

If you intend to totally support the visit both financially and with accommodationthis here is a listing of fairly typical supporting documents that you should provide:-

- 6 months Bank statements
- 6 months pay slips
- Proof of address (eg utility bill, Council Tax etc)
- Accommodation details (including owner's agreement / rental agreement)
- Sponsors passport copy (Photo page, any relevant stamp pages)
- The main reasons for wanting the applicant to travel at the specific time

Good luck