It will come as a terrible shock, after all the efforts and preparations you have gone through and all the waiting, to find your application for a visa has been turned down. Now is the time for a cool head and rational mind, as you have some big decisions to make about your future prospect.

First read and re-read the content of the rejection letter. By law they have to give specific ground or grounds for rejection. While the language can be technical and convoluted, it should spell out the exact reason(s) why you have failed to meet the requirement.
If there is a simple error of facts, such as claiming you haven't sent in a particular document when you have, you can try complaining first. Your comment will be passed to the relevant visa office and they may then decide to issue your visa. See

https://www.gov.uk/government/organi...ints-procedure

Secondly find out whether you've been given the right of appeal. Not all types of visa come with the automatic right of appeal, though you may be given administrative review - to ensure that proper procedures have been followed. You only have 28 days to lodge your appeal, so don't dither. You have to pay a fee of £80 for paper consideration or £140 for tribunal hearing. Most opt for paper only. You need to address every reason they have given for refusing, and show they have been wrong in rejecting, or supply additional documents - available at the time of original application but somehow got left out. They only want copies, not originals for the appeal.

Thirdly, every appeal process first goes through review by Entry Clearance Manager (ECM), who will look at your application afresh and see if any errors have been made when processing it or they have misinterpreted your situation. If they then decide to overturn the decision, you will be told about it and asked to submit your passport for visa or vignette to be affixed. If the decision isn't overturned, your case goes to the First-Tier Tribunal. As there is a big backlog of cases to be decided, there is a long wait, often up to a year until they fix a date for the judge to examine your case or a hearing. The tribunal hearing will take place in a room, and although it's a court of law, the atmosphere is more informal, and if you aren't legally represented, the court officials will try to be as helpful as they can for you to state your case (if you've opted for a hearing) but they won't give advice. Home Office, if they are contesting the case, will be represented by a presenting officer, usually a solicitor. While the judge may give their decision there and then, it's normal for them to reserve their judgement, giving them time to decide on the outcome. You will be told about it by letter. If your appeal is upheld, Home Office is given a short time to decide whether to lodge an appeal to the upper tribunal, and then the judge will decide if an appeal can be made. The same holds true if you lose your appeal. If you win and Home Office decides not to contest it, you will get a letter or email requesting your passport for the visa to be attached. Often you wait rather a long time for this, and a delay of several weeks, sometimes months, isn't uncommon. You may get your appeal fee back.

It's permitted to make a fresh application while your appeal is pending, and then withdraw your appeal when you get your visa.
Slightly different procedure is followed if you appeal from within UK.

https://www.gov.uk/immigration-asylum-tribunal/overview

Often the best course of action after a refusal is to make a fresh application, ensuring that every ground for rejection is properly addressed. This is usually a quicker route, though you will have to pay the full fees again (you will get your health surcharge back if you've paid when you are refused).