Team picks the entire lane layout already and you are either stuck with support or picking another hero/champ all together. The ADC decides to post "a support plz" so instinct should tell you ok I will support you. Now that the match begins you notice something strange the person you are "supporting" is missing easy last hits and decides to play way too aggressive. This is when panic sets in because A) Your lane partner is completely awful or B) They have terrible internet yet still decide to play the "carry" role. So to clear things up this match is going down high fast when you see that you are slowly losing the lane even when your jungler decides to gank when the wave is completely pushing but that is a post for another time. Back to that oh so important "carry" lane partner. There are rules you should follow when you pick a carry and decide that "I want to be the main attraction of this match" yet cannot actually do the basics of what a carry should be doing to actually be effective during the mid to late stages of the game.
Let's start with the basics if you want to carry LEARN TO LAST HIT before you even start a match LEARN TO LAST HIT if you cannot LAST HIT you are doing it wrong. When you pick your items and you know you can't LAST HIT well then you should decide on items that will help you LAST HIT. Just in cause you didn't get my point you should LEARN TO LAST HIT because if you can LAST HIT you will get X amount of gold over the lane not seem like a complete noob when you decide "I can fight this battle I was able to free farm all match" yet you only have less than 20 cs by 10 minutes. You should be able to last hit at least half the creep wave in Dota 2 and 3/4 in League of Legends... for obvious reasons if you don't know those obvious reasons you need to take a look at the difference between denying and last hitting in Dota 2... or in other words Dota 2 is around twice as hard to last hit in because of having the ability to deny creep which gives half exp and no gold.
Now that we got the whole reason to learning how to last hit now we can get even simpler before you do anything open up a bot match of the MOBA of your choice and play 10 minutes of the match..... go on..... now that you have finished the match look at both your Last Hits (and Denies for Dota 2). Write that number down on a sheet of paper next to you and open up a real match and focus just on last hitting (and denying) don't worry about anything else and if you have a lane partner record their last hits (and denies) after the match. If your lane partner ended up with more last hits than you.... you failed your job as a carry it doesn't matter how many kills or assists or deaths you got. If your support (the person who is just protecting you for the first few waves and getting close to 0 CS) gets more last hits than you.... you failed your team even if you won the match. You were a liability to your team because chances are you got less Last Hits than the enemy in the lane against you. This opens that lane to be a potential threat or the cause of your lost because you weren't able to last hit more than that lane causing them to be more farm and having the items needed to win fights.
You are likely now saying "BUT BUT BUT I won the match it doesn't matter" but then you are wrong at any point in time this just means that either a jungler or another lane easily won their lane to help you out when you didn't win that lane. Yes these games are team games that require help from other lanes but if you actually work harder to become better you wouldn't have to worry as much because you are certain you are winning your lane for your team and don't need a gank or help from other lanes. When you win your lane you aren't just helping your team out you are making sure you are putting your support to full use. Even if they decide to just sit there and soak up experience. Now remember that number I told you to write down from your bot match. For Dota 2 it is around 120 lane creep within 10 minutes if you are playing in a solo match with no bots you should get around 80-100 easily if you are not well not you have a number to work toward. On League of Legends it is around 100 so you should easily get 80-90 because you don't have to worry about denying. I make denying a big deal because if you have to worry about twice as much creep you are working harder. Dota 2 200 per 10 minutes LoL around 100 per 10 minutes that you can Last Hit and Deny so Dota 2 takes the cake for having more awareness.
Now think of it from this standpoint if your lane partner can get more CS than you after the match is over that means they clearly can farm better than you. You can either expect that statement or not but chances are they are and you are more likely ruining their match when you decide to carry and can't last hit. Even if they are the nicest person to you chances are deep down they are saying "what a waste of time" for ever easy last hit you miss. If you are that type of person who cannot last hit don't try and carry chances are you are ruining it for someone that has to support you and you decide to be horrible at your only job during the first 5-10 minutes of the match. What you should do is the next match you queue up play as a support for once and learn how it feels to be completely reliant on your lane partner to last hit and watch them closely because you are doing the same thing when you decide to carry and cannot last hit.
So now you see the basic rules for asking for a support and what you could be causing your team when you decide to not be able to do your job. Now we can make you better if you decided to actually read everything here because you actually want to carry and be good or at least better than what you have been doing. Start a solo match with no bots and keep practicing last hitting and as your number go up go into a bot match and practice with them now that you last hit for 80-90 by the 10 minute mark go into a real match. Yes you will only get around 40-50 I know that because now there are real problems you have to deal with but now you aren't getting less than 30 last hits by the 10 minute mark. Keep your head on just last hitting your lane partner is the only person that needs to worry about a lot more. From warding to checking on ganks to protecting you from getting those last hits. Your job as a carry is to farm and farm well if you aren't farming well you are not helping your team to win no matter how good they are doing you are causing your lane to be lost. Once you are hitting 60-70 last hits my job is complete and now you should easily be able to carry your team and win your lane due to the free farm that you obtained. Now go carry and play as a support once in a while