are you guaranteed a place if you meet all physical and mental requirements?

Once you are given a date to attend the selection process, you will have to meet the physical and medical to pass, amongst other things such as your own personality and cognitive ability; all the standards/test you will be required to carry out are on the British Army page.

If successful at selection, you will be given a start date for your selected job choice if available. Are you looking at HR Specialist in particular?

I can't give you a guarantee, however if you pass selection there should be no reason for you not to get a start date for a job of your choosing.

i am having trouble applying, is there an email i can contact to help me?

https://apply.army.mod.uk/login

Have you made an account on the candidate portal? Once in, you will be able to apply, talk to your recruiter and follow the progress of your application.

You can also give your local career's office a call for more info.