Last hitting in Dota is hard, but remember. Its relative to your current skill. One of the things in my early Dota career that I would religiously keep track of for myself was my last hits at 10 minutes. There isn't a target number that is good or bad, but you do want to know if you're performing at your current average, above it, below it. Also taking into account the lane matchup, and how interfered or lack of help did you have. There are a lot of variables here, but the idea is that you become able to reach your average in a variety of situations as well as increase it through time. From free farm, to highly contested you've just gotta give yourself the right grade for that number. If you have a few hero's you like, be certain you know your performance over time for those specifically. The other very strong reason to do this is because even your losses will likely give you 10 minutes of even play. You can't really compare your 30 minute/game end last hit numbers as simply because the variables are so broad.
I only have three item related tips that you should think about at all at your current level. The first is this. If the enemy team is almost all physical damage, buy at least one item that also gives you armor if you are not an agility based hero. Many good items have an armor component. Mek, Shiva's, Lotus Orb, Medallion of courage + upgrade. AC, Crimson Guard etc. Second, If the enemy team has a great deal of stuns and magical burst, force yourself to buy a BKB at the 30-40 minute mark. These two choices are easy to conclude and give a lot of value. Last, and maybe the most important tip is to buy an upgraded magic wand on ever single hero you play until you have a justifiable argument as to why you shouldn't buy one. +4 stats plus an active that gets you started from early in the game using inventory slot actives. I know it might seem an odd suggestion, but 17 wand charges will win you fights, get you that last bit of mana for a spell you need to finish, and start building your active item use muscle memory that is beyond the skills of your hero. Many items in Dota are very strong, but only if you click them and use them like your 5-6-7-8th abilities. The wand guarantees that you buy a cheap active, that won't retard your long term item progress, is great in early fighting, and can be bought in the first minutes of the game every game. You can place it on your favorite easily accessed inventory hotkey, and it transcends the game or hero that you play. If it saved you once or got you a kill it has paid for its rental fee of 230~gold after you sell it due to lack of slots later in the game.
I do have a 4th item tip that crossed my mind. If the enemy has a Shadowblade(s) or an invisibility hero and you have an inventory slot that is going to waste. BUY DUST. Even if you are a core. When you gank someone with a shadowblade, dust prevents escape. Detection is poorly bought in most low tier games. The worst thing about that, is that by the time people decide to buy detection they have been killed 6 times in the same way by the same hero. If you start here at least, that will set you on the road to understanding sentry use in the future. If you tunnel an invis hero with dust constantly it pays for itself and will make that hero so bad it will do nothing later in the game. Keep in mind, you might be the first person to actually actively counter the invisibility picks of your opponents. You will get to abuse them because they will have to learn how to avoid being killed by dust to stay effective into higher ratings of skill.
Your third point is the knowledge cliff of Dota, you don't need to study shit like school work but when you get ruined by a hero you don't know, do take the time to read his skills. If how that hero does damage doesn't make sense, maybe go to a Purge guide, or ask here. This can only be learned really by doing. Even knowing all the hero skills by memory does not give you the insight on how the best players will use those skills to kill you.
More walls, I can't help it.