I don't think 5kgs will be enough to bulk up any muscle, but it will help you get tight... perhaps after a few months or so you might consider investing in some heavier weights if you like lifting and want to develop a bit further.
Here's my suggestion for a basic plan:
Shoulders:
Shoulder press - Stand upright, and hold the weights in your hands parallel to your shoulder, palms facing forward. Then, lift the weight up over your head, straight up, until both arms are extended fully upright above your head. Then slowly return the hands/weights to their starting position at about shoulder level. When doing this, you should feel it in your shoulders. You should be able to do a heavier weight for this, try the 5kg.
Shoulder lifts - Stand upright, weights in hands with your arms DOWN at your sides. What you are going to do with this is the same action as a jumping jack, but you won't jump. Keeping your arms locked straight, palms down, LIFT your arms to your side until they are at about shoulder level, then slowly return them to your side. This exercise uses a lot of leverage for resistance, start with the 2.5kg.
Chest:
Bench Press:
Lay flat, face up, with the 5kgs in your hands, resting right above your chest muscles (your pecs). You want to lift the weights STRAIGHT up, RIGHT above your chest, PRESSING them up into the air.. and then return them to chest level.
Fly:
This requires a narrow bench of sorts to lie back on... this exercise is similar to the press, but instead of a straight up and down motion, you are going to be moving your arms as if you were a bird flapping wings. Start with you laying back on the bench, face up, and with the weights in your hands, rest them right on top of your chest muscles. Now, lift them a few inches up so they are "hovering" right above your chest... your arms should be bent... and, KEEPING that same bend in your arms, locking your arm bend, slowly let your arms fall to your side, letting gravity rotate each weight away from your chest, and your elbows rotating until they are pointing at the floor on each side of you.... then, KEEPING your elbows locked at that same angle, slowly rotate them upwards, bringing the weights back together, directly above your chest.
Back:
Fly:
Same exercise as above, but do it FACE DOWN. Start with you laying face down on the bench, hands/weights literally just above the ground on either side of you. As you pick the weights up on either side of you, you should feel the muscles between your shoulder blades carrying this load. Lift them until your shoulder blades are fully contracted, then slowly release them again, lowering the weights to just above ground level, then repeat.
Since all you have are dumbells, you might want to try this exercise at different angles so as to "feel" the different ranges of muscles from your upper to lower back working.
Biceps:
Curls - for the FRONT of the arms. Sit on the edge of a bench or chair, and spread your feet so there's space between your legs to manouever. One arm at a time, lay your elbow against the inner surface of your knee with the weight in your hand. Let gravity lower your hand/weight until your forearm is straight, pointing towards the ground.... and then moving your forearm only, keeping that elbow firm against the knee, "curl" it by lifting the weight upwards towards your shoulder.
Triceps:
Dumbell French Press: Sit upright on the chair/bench. Take the weight in one hand, and lift your arm STRAIGHT up into the sky, palm facing foreward. Keep the base of your arm locked and straight, but allow gravity to let the forearm ONLY drop so that your hand slowly curls around behind you and ends up with the weight just behind your shoulder... at this point, use your tricep (BACK of the arm) to LIFT the forearm BACK until your entire arm is again pointing straight up to the sky. Then repeat.
I would start out doing one set of 15 repetitions of each of these exercises for the first 5 workouts... then work up to two sets of 12 (do twelve continuous repetitions, then take a break for a few mins, then do another 12)... then after another week or so, work up to three sets of 10 repetitions.
Lastly, you really need to work your abdominals... and you can do this doing sit-ups or "crunches". This would complete a basic workout package for your "core" given the hardware you have to use.
