As @DumbCoder mentioned, the burden of proof is on you to show that you have an strong and valid
intention to leave. This could include things such as family back home (spouse and/or children), a large bank account, a really good job, some combination of the above, or something else that ties you to your home and offers a compelling reason why you would leave the UK and go back.
Immigration officials are very wary of young solo travelers (especially young women), because they often don't have strong ties back home and thus sometimes have very strong incentives to overstay their visas.
As a further example, this is from a recently archived Home Office form (note the first listed item in particular):
Grounds for doubting the applicant’s genuine intentions to visit the UK
This is not an exhaustive list but may help with your
- the applicant has few or no family and economic ties to their country of residence, and has several family members in the UK
- the political, economic and security situation in the applicant’s country of residence, including whether it is politically unstable, a
conflict zone or at risk of becoming one, which may lead to doubts
about their intention to leave the UK at the end of their visit
- the applicant, their sponsor (if they are visiting a friend or relative) or other immediate family member has, or has attempted to,
deceive the Home Office in a previous application for entry clearance,
leave to enter or leave to remain
- there are discrepancies between the statements made by the applicant and the statements made by the sponsor, particularly on points where
the sponsor could reasonably be expected to know the facts but does
- it has not been possible to verify information provided by the applicant despite repeated attempts to do so
- the information that has been provided or the reasons stated by the applicant are not credible.
So your best bet is to find as many reasons as possible for why you would return to your home country, document them, and provide those documents to the immigration officer.