In reply to Rachyjane:
As most people suggest, just more climbing is all you need.
But if you can't.... For a beginner some doorframes could provide an edge that is way too small, especially if painted / varnished, so don't be surprisedby 2 secs. Hang board training is often just up to 7 secs on, 3 off, repeat.
Any idea how deep it is, the edge? If u really can only get out climbing once a week, fingerboard can help start getting your fingers strong, but be very careful if you buy one , don't use any small edges, maybe even close them off with tape so you can't risk injury.... Nearly all the edges on most fingerboards are too small for beginner cimbers, and are meant to be training aids for intermediates and above.
I would have thought you would want at least 50mm depth.
The beastmaker 1000 provides some good jugs, try doing pull ups on these, or a bar, or sanded slightly rounded beam, that you can get almost the full length of fingers on top of, that may be 50 - 70 mm deep depending on handsize. You will be hanging on a big hold, it will still develop your fingerstrength, and pulling up strength. Round off the edge, it will also help if it has some texture, not paint or varnish.
Perhaps have a foot edge / big strong theraband loop to put your foot in / chair to assist if necessary.
Maybe 3 to 6 months in, or longer, you could start playing with edges that allow all your fingers to the mid joint, that's probably still about 30 to 50mm deep.
Always warm up, read about the dangers of crimping, and different crimps, and avoid using them as a beginner, but crimping holds is still recommended at times and is a grip you need to train, later .
Read about how to hang right, with bent elbows. Theres lots of web trails to read on fingerboarding, and injuries. Read about how to hang right without fully stretching your shoulders too. Don't drop into fully stretched shoulders, put feet down first.
Read lots about injury potential... Your best tip, get working through all the thereaband injury prevention exercises, buy a variety of bands, the cheapest sports direct stuff and argos davina or pro sport stuff are well worth incorporating straight away.
Have the board or bar at a height you can reach without needing to jump to it, to avoid shock loading vulnerabilities, also so that when your fingers start to uncurl you can touch back down to ground with your feet and then release grip, rather than slip off the hold with weak grip.
Maybe you will manage to use it regularly,... In 6 months to a year, when you start to use the medium size edges, expect slow progress, be patient, there's plenty of time, years to build it up, before going to, <25mm edges.
Train other large sloping holds too, long before you play with the small edges.
Some people do start out with more fingerstrenth than others.
If you are a teenager or younger, read about why you shouldn't be doing it at all.