Couple of quick strats.
Side tilt->jabs should be some of your most used attacks on the ground. Side tilt perfectly combos into jabs which gives you some nice damage.
UTilt gives you the perfect tool for taking it to the air. Depending on how much you use it and your opponents percent you can easily get two utilts before they are thrown high enough out of its range.
Get used to the halfway mark on Withdrawal. DONT use it all the time, but if you can space a hit, you can surprise your opponent by withdrawl->stop right under or by them->utilt->whatever. Withdrawal can play some nice mind games if you can space it right, just do not use it to often as becoming predictable with it is entirely punishable.
SHing NAirs should become one of your bread and butter ariels. Unlike fair, Nair isnt one of your kill moves and is pretty decent as an attack. SHing and almost instantly nairing will give you no lag when you land. NAirs are a fun damage dealer
You can do the same thing with DAir and it can launch, so its ok if you get used to DAirs to use that in the same way.
FAir should saved for kills if you can. So, to save it, you should mixup your nairs with RARed BAirs(running and hitting back just before you jump so you jump facing the other way). If you are way up in the air and you opponent is in your front, use nair, not fair. For one, you dont want the launch back if you are still trying to damage then, for two, you want fair to keep its knock back if you want to kill easily at higher percents.
DO NOT DASH GRAB IF YOU HAVE ANY DOUBT YOU'LL LAND IT. Grabs are great, and DThrow can help kill, but squirtle has amazing lag when he wiffs a dash grab.
Learn how to climb the waterfall. It's slight, but knowing how to control that can help recovery and can help you get hits with that last hit of waterfall that launches. It can also help you land all the hits of waterfall when you use it. It's in your best interest to learn how to control it instead of letting it do the same thing everytime.
Shellshifting is a must to "Master" squirtle as he is now. It gives you your easiest spacing technique. Get used to stalling and dashing out of it. After you got the spacing games down, start getting used to hydroplaning and its derivatives. Hydroplaned USmashes and FSmashes can be used as kill moves and gain a lot of utility from the slide. USmashes are the easiest to pull off and land, so you'll see a lot of squirtle players use it. Hydroplaned DSmashes will equate to some good damage if pulled off right. Also, get used to tilts out of the shellshift, as sliding utilts and ftilts will allow you to combo more. All in all, this is the most advanced aspect of squirtle and should only be learned once you get the basics down.
Few more notes. Fatigue really only effects Squirtles kill chances. If you save his fair or his usmash you can still kill during fatigue. Squirtle shines at racking up damage with tons of small hits and fatigue just helps with that as it drops knockback a little. His combo ability increases, so switching out of squirtle just because hes fatigued isnt always the smartest thing to do. It is smart to switch due to damage though as he is a lightweight:O
