I moved over to PC gaming 8 months ago, since then I've just been playing sandbox, RTS games as well as RPG and action games with a controller.
I bought Titanfall recently and I play with a keyboard/mouse and i completely suck at it. Seriously, my ass gets handed to me each match. I cant keep up with the fast paced action, I always press the the wrong button which means I get killed immediately during a firefight.
Back on console I used to be good at call of duty or battlefield. You might wondering then why I dont use a controller on my PC. It's because for some reason it doesn't feel right. It's actually worse than keyboard and mouse for me. On the PC, the xbox controller is not precise is enough - even when I adjust the sensitivy. That's just me though.
I don't have a mechanical keyboard because I can't afford it. I use a membrane one: Zalman ZM-K200M. I use a perixx gaming mouse which is pretty sweet got it for £12 on amazon.
Kewldude007 pretty much said it. You just have to become accustomed to how your mouse moves around the screen. I think Counter Strike bots would make a great starting point. Try to make it challenging though. Don't set them too easy otherwise you're not putting yourself in compromising situations and learning how to compensate. Other than that though, yeah you just have to practice. And the only way you're going to get better is to get your ass handed to you for awhile. There's no quick and easy way to get good, unless you use hacks such as aimbot and godmode. But there's no fun in doing that.
As Kewldude and Mnemic said, practice, practice, practice. Although it might seem a bit odd, exercise helps a lot when it comes to FPS games. This plays into the twitch muscles and hand-eye coordination and whatnot.
You just have to put some time and sweat into it. The jump to keyboard and mouse is not easy. In fact I actually did most of my KBM training in Skyrim before doing and FPS games. But if you want to adjust to it the fastest, I would say go for CSGO or CSS (I prefer CSS) and do some bot matches. You should be able to adjust to moving the mouse to aim within a week.
Practice makes perfect. I did have some PC gaming experience from youth. On my return to PC, I found the Max Payne 3 single player campaign useful, did a good job of getting me reacquainted with keyboard + mouse.
I would definitely say get CS:GO and practice with bots. Because CS:GO will teach you to aim for the head accurately and compensate for recoil. Yes it is a Hardcore FPS, but it's not hard to learn. When you buy it, just go into Offline with bots>deathmatch>terrorists>AK. Once you are comfortable enough with your skill, raise the difficulty of the bots. Keep going until you're against expert bots. Once you can win against those on a regular basis, you're definitely ready for most FPS games.
The reason CS games get labeled as "The most hardcore FPS" is because the only thing keeping you from being the top player, is your skill. You don't start the game at any disadvantage, the best and worst players are on the same level game wise. Again the only difference is skill.
Practice always makes perfect when it comes to getting better at a game.
Titanfall isn't a good game to start with. Because you have to play one teams games before the other that means the one side is always more experienced.
CS:GO yes, but play bot matches.
TF2 would also be good as well. It is a little slower paced and classes like Engineer, Pyro and Heavy as well as the Sniper don't require too much control changes and should be a good way for you to practice.
Never use a controller again when playing fps games and try playing campaign modes first on fps games. campaign modes are one giant tutorials to get the feel of a game. also trying upping your mice dpi settings or if you dont have a dpi switcher, try to heighten the mice sensitivity so you could move faster. pratice pratice practice. maybe 10hrs worth of play will make you a better player
turn mouse accel off and lower your sensitivity in game in fps games I would say 8in 360 is a minimum I play with a 17 in 360 personally. Just practice it takes a lot of hours to get the muscle memory down. Took me probably 100 hours of in games time to get good.
Go into windows mouse settings, find 'enhance accuracy/precision' or whatever, disable it. In every FPS game you have you need to find the mouse settings, and untick or slide mouse acceleration down to 0. This makes your mouse feel like putty and means that movement distance is correlated to movement speed. Which makes it hard to play FPS.
This should help alot, when I launched CSGO lately the mouse felt like putty, it was ridiculous. Also CSGO is bad, it has horrible hit registration, I am not just saying that because I can't hit, but because also when I do get head shots and whatnot, It was more like a chest shot/leg shot.
do not play bots in CS:GO.... ever there are many guides on youtube for CS:GO, but for FPS games in general, I would say that you should look up reflex testers and accuracy testers on google and play those. the more you practice quickly moving to one place to click in those the more you will be practiced for in game. Doing this for like 10 minutes prior to playing any FPS is how I used to do it. I now have no need to do it.
getting rid of mouse acceleration will help build muscle memory quicker. Some games don't use raw mouse input so here is a fix I use. be aware that this is a registry edit but it wont break anything, I do this for my gaming pc
Also in mouse setting uncheck enhance mouse pointer precision. Secondly to help your aim keeping is consistent across multiple games helps. You will have to change settings in configs but the site I recommend will help.
http://www.mouse-sensitivity.com/
It helps to use a lower sensitivity, I feel comfortable with a 14.6cm turn when hip firing for 360 degree turn in BF4. Most counterstrike players use much lower. Lastly to help with recognising keys I used to put bit on masking tape on my WASD keys to help until I got my corsair k70. This helped greatly and I am now playing much better than when I was when I first started and my Stats keep improving. Lastly watching some Major YouTubers will help for thought processes while playing in game I recommend Rival XFactor whom I learnt a lot from I hope this helps