Whats your finishing like? In all honesty a striker is a very vauge position because there are so many ways to play it. I wouldn't call torres or ronaldo de lima the most aggressive or strong strikers, but they play brilliantly because instead they use their pace and positioning to make sure they don't need to muscle for the ball.
Whereas a players like drogba and ronaldo thrive on chances in which they can jump up and muscle for the ball because they are very tall strong and aggressive. To be honest, been a striker myself and playing with people much older than me, half the battle is not been scared of getting clattered, i always had this fear about jumping up for headers because i thought i would get punched in the face by the keeper or head butted by another player, but if you go in aggresively and be strong keep your arms wide to defend your body it really is unbelievably how many times you will win a header.
Back on topic though, if you find that your getting pulled back by defenders as you try to run past them, then don't be afraid to go down. sorry to say it but some of the defenders ive played against are truely disgusting and will tug your shirt if you make it past them, so if you think that your not going to be able to wiggle your way away from them, then you should not afraid to go to floor and get them sent off for been the last man and taking you down.
As for positioning, you want to be in space, if you inspace then it means that no one is marking you so your not going to be challenged as you go to control the ball, and also it means you will have more time once you've controlled the ball to shoot/pass/dribble.
What i do in a situation is i pick my defender who im going to be paired with by walking up to him and standing right on his solder onside so that if a ball is played i can recieve it without commiting the last man foul. Then as the play builds up you slowly drift off your defender quietly and sneakily. It sounds stupid sneaking around in a football match but honestly, once im playing i don't talk at all or give my position away to the defender untill i want the ball. Slowly as i drift more and more off the defender into space i wait untill the correct moment. Then i either raise my arm indicating to my team mate i want the ball, point ahead of my in the direction i want the ball to be played through, or yell for it and indicate wether or not i want to feet ahead.
99% of the time i prefer the final ball to feet because its much easier to play a ball to feet than it is to play a perfect ball through, also with the ball through, if their sweeper is good you will often find the ball getting taken away and cleared before you can make it to it. Where as if i get played to feet then i have a chance to control the ball properly and run with it myself. The only time i ever want a through ball is when we are on a fast counter attack in which everyone is running.
All there really is to positioning is making sure you keep moving around, keep moving away from the defenders and getting into any space that you can see and then call for the ball, as a striker that is all you can do, then if your team mate plays the ball to you, hopefully you've picked your position well enough to have time to dribble or shoot and hopefully score.
An essential part to striking id like to include is that you are not limited to the back line. Alot of strikers seem to think they should constantly be on the last man and waiting for the ball over the top, this is not entirely correct. The game of football is divided into 3 sections, defense, midfield and attack, now the transition of these stages happens very differently, defense to midfield is quite simple because there is less pressure on the man on the ball so they can make quite an easy pass. The midfield to attacking transition however, is a very different story. The whole purpose of attacking mids and wingers is to bring the ball from midfield to attack. What you will often find as a striker however is that you need to drop back for the ball, often you will find MASSIVE gaps in the pitch just between the other teams defense and midfield, because the midfield is trying to close down your midfield, and the defender are trying to get back to contain any chance of attacking play.
So if you manage to drop back into this gap in the pitch, you will more than likely create a seriously dangerous attacking position for your team, all it takes is for you to drop, pick up the ball in the gap, turn, look up for your other striker and then bang, ball over the top and he runs onto it and scores.
The wings or channels as some people call them are also a highly useful place as a striker. During the course of the game, the full backs often will be involved further up the pitch, this leaves huge ammounts of space in the defense. If you move across into one of these empty patches behind their full back then you create a seriously dangerous positiong for your team to attack through. Infact your presence alone in this location can drag one of the centre backs across to mark you which leaves massive holes in the middle of the defence and can result in your team overwhelming their remaining defender. If however your not marked and the ball is played to you, then you can catch all of their defense in no mans land as you sprint forward which leaves them no chance of catching you as they come across.
I really hope this post helps, ive spent YEARS playing as a striker so ive comed to learn all these little pointers, if you would like to know anything else honestly, PLEASE do ask, i would love to help.
BTW, i didn't mean to write an essay, got a little carried away lol..