you need to be strong for rugby - do you just want stength or do you want size as well?
Find an online calculator of your calorie maintenance line and then eat 300-500 calories more than that a day-bulking. This will provide your body with enough excess energy to make muscle. Also, about 1-1.5 grams of protein/pound of body weight per day.
If you just want strength maybe a 3-6 rep range. If you want a bit of both maybe 5 sets of 5 reps (5x5). Remeber, keep to compound workouts for muscle gains. Use mainly free weights, often machine exercises aren't very good. ( I know, a generalisation.)
Look on sites with videos of all exercises e.g. criticalbench.com or coopersguns.com
You will not be able to gain size immediatley. Be patient, it takes some time and you will get better at working out and eating big. Carry a piece of paper around with you and write the calories and protein from each meal. I go to school so can't take the suggested 5-6 meals a day. Just eat big breakfast, eat lunch and maybe bring a sports bar or two to school with you then have an early dinner then maybe another one later on.
I suggest probably a full body workout (fbw) 3-4 times a week with one/two days rest inbetween. If you are struggling for calories or protein I suggest milk - its great for both.