Welcome tot he forum!
It takes a while to get used to the differences between the four main summer visitors that spend most of their time wheeling about catching insects in the air (OK, well, I admit it, it took me a while!)
Swifts appear all dark and very sickle- or boomerang-shaped and are comparitively large compared to the three hirundines below
Swallows are mostly pale from below and all dark above, with a dark (and reddish) throat/head and long, dark tail streamers
House martin are very white below and dark from above, with much shorter, dark tail, and a prominent white rump
Sand martin are pale from below with a dark throat band, but noticeably brown-coloured.
Also, with a bit of practice, you can recognise differences in the flying style: House martin flight is more fluttery than a swallow which is faster, more direct and swooping, while sand martin are more fluttery still, and swift are very fast and look somewhat stiff-winged to me.
Finally, the calls are all quite distinct: swift calls are quite loud and screeching, while swallows are quite twittery and varied. House martin and sand martin calls are quite similar, less sustained than swallows possibly, and more chirpy and clipped, with sand martin being slightly scratchier or hoarse sounding (IMHO!)