Snowboarding and Skiing

　Be sure to remember the preposition when you are saying where you are. In this example, without "at" you are saying you are a mountain. :)

　In this sentence snowboarding is a noun so there is no need to add "to do."

Based on the rest of your essay a way that may sound better would be: "I tried to snowboard".

When you say "I tried to snowboard" there is a slight connotation that it didn't go well, or that something bad may have happened. Other examples: I tried to eat the pizza. (But it was too hot and I burned my mouth). I tried to take the bus. (but I was too late and I missed it)

"I tried snowboarding" is neutral. It could have gone well, or it could have gone badly.
Example: I tried snowboarding and I loved it! I tried running but it was boring

　Because you are talking about your experience you don't need 'about'. (If you were talking about your knowledge of snowboarding you would: "I don't know anything about snowboarding)

'I tipped over many times' is correct; however, 'I fell a lot' is much more common and natural in this context.

There is no need to add 'to be able' here. The verb 'managed' in this context includes that idea with it. I managed to turn = (It was difficult but) I was able to turn

　This should be separated into two sentences. The first sentence is correct as is. But I don't know if it is what you mean. "I felt a severe muscle pain" means that there was a specific muscle or part of a muscle that had an injury that hurt a lot. That may be exactly what happened, but based on my own snowboarding experience I think it is more accurate to say, "I was very sore." To be sore, or to have sore muscles is usually how you feel after exercising very hard. You get sore all over, or all over the muscles that you used in the exercise you were doing.

The second sentence is understandable and native English speakers would be able figure out what you meant, but it has a conditional clause so I made some adjustments to help you.
"I thought that I couldn't..." means that you thought that you were not allowed to do something. If there was a law that said you couldn't snowboard 2 days in a row. Of if your mother told you that you had to stay home. Because a temporary condition was going to stop you, "I thought that I wouldn't..." is the correct phrase. You could've still gone snowboarding if you really wanted to even if you were sore, or you may not have been sore today at all.

　"Easy" and "hard" are used to talk about the amount of damage or difficulty an activity (or person) causes to an object (or person). Skiing is hard on the body. Swimming is easy on the body. My boss is hard on me (he makes things difficult for me). My mom went easy on my brother (she didn't punish him very much). He is hard on his car (he doesn't take care of it)

　The adjective "first" in this sentence requires a countable noun to modify, so you need to add the noun 'time'.

The word somewhat is correct, but in this sentence it doesn't sound very natural. The phrases "sort of" and "kind of" are used all the time in situations where you know something about the subject, but you are not an expert or don't know many details. I sort of know how to type. I can kind of speak Russian. I sort of know what you're talking about. (in speech they get joined together and sound like sorta and kinda so they are sometimes written that was as well)

　I'm not sure what you meant here so I guessed. The word fun always requires a verb (to have fun, to be fun, to look fun). And, like in the first sentence you need the preposition to clarify your meaning.

(Here you need the article "a" as well. In most cases if you are "at" some place you need to include either "a" or "the". There are some very important exceptions to that rule: I am a home. He is at school. You are at work. They are at church. We are at Steven's apartment, They are at my brother's house. (This issues gets a little more complex, but 95% of the time you don't need "at" with: home, school, work, church, or any place that belongs to a specific person)

　There are 3 options here that all mean the same thing, but they each have a different connotation.

"I will" - This sounds very formal, or very determined. "I will ski tomorrow, and no one will stop me"! or "As indicated in article 2 of the court record, I will ski tomorrow."

"I'll" This is better, it's more natural, but it can also sound dismissive. "I'll ski tomorrow, I don't want to today." or "I'll ski, but I'd rather have a different option"

"I'm going to" (or "I'm gonna" in spoken english) - This is the best option in this context. It's the announcement of your plan to ski that is the most neutral (not too formal, not too dismissive). I'm going to ski again tomorrow because I don't fall down as much when I ski." "I'm going to ski tomorrow because I had fun today."