The best advice i could give is ask where you buy your fly lines they are usually helpful
and always buy the best lines and leaders you can afford the cheaper ebay type lines lay like a snake back and are very hard even for an experienced angler to retrieve with any chance of a take presentation same as any type of fishing is paramount also match your rod weight to the line
not sure if you need it but maybe the following explanation of weight ratings may help some beginners
The coding of numbers on fly lines is a system that is used universally to determine the casting weight of fly rods, a bit like the test curve rating used in course fishing.
First I'll explain the letters.
WF means weight forward and as the name suggests is a line where the majority of the weight is at the forward part of the line, this part of the line is thicker than the rear of the line and is what loads the rod when casting.
There are other lines where letters are involved like DT which is a double taper line and SH which is a shooting head.
The numbers indicate the weight of the line, the lower the number the lighter the line.
When purchasing lines you should match the line to the rod, all rods will be marked with a AFTM number (Association of Fishing Tackle Manufacturers), this is the weight of the rod, for example the rod may be marked AFTM 7# the ideal line for this rod would therefore be a 7 weight line. likewise a rod marked AFTM 4# would be suited to a 4 weight line. Some rods are marked with dual ratings for example AFTM 6/7# this means the rod is suited to both a 6 and a 7 weight line. When rods are marked with a dual rating the general rule of thumb is that when line is matched with the higher rating more distance would be achieved than with the lower rating line and when matched with the lower rating better presentation is achieved than the higher rating. I hope I have explained this well enough