Firstly, decide what sort of tomato you want to grow. Cherry varieties get ready before larger types and have a more concentrated flavour. Large varieties such as plum and beefsteak are trickier for beginners. I recommend:
Shirley F1 AGM: very early, short-jointed variety suitable for heated or cold glass/polytunnels; heavy yields of top class fruits; resistant to TMV, Cladosporium and Fusarium.
Sungold F1 AGM: golden-orange cherry – good yield of attractive round fruit; good flavour
Sweet Million F1 AGM: red cherry – long trusses of sweet fruit; good yield
Tumbler F1: early bush variety for baskets & tubs; red cherry-sized fruits with excellent flavour
I have grown all of the above varieties outdoors in East Anglia.