Congrats, you are emarking on a 20 year career, either with your created player from Success Mode or one made up on the spot. (or an already existing big league player, but whats the fun in that?) Here are a few time to help with breakin into the bigs
1- Earn your managers trust. The sooner you do this, the better youll fare when you run into 0 for skids.
2- Save often.... saving is never overrated in this game.
3- Get used to being in debt after your first year, especially if you are good. Your Manager messes up your donation at the beginning, adding a zero at the end ($200 becomes $2000), if you are good enough to put yourself in a big hole (i was -$250,000 after my rookie year as a pitcher) then you are probably good enough to land a big contract anyway.
4- Batters adjust to you if you are a pitcher, pitchers adjust to you if you are a batter. I noticed my first character was a pitcher, and my first year i posted over 300 Ks, and had a 23-4 record. Second year i went about 22-6 but only 150 Ks, the batters hit me more often.
5- YOU control the game, if you want it to be harder, up the pitch speed, or start your My Life at a harder difficulty. Personally i enjoy hitting .470, its surreal
6- Dont like your position as a pitcher? Talk to your manager. If you are a starter but want to close games, or the other way around, talk to them and they will work to switch your role!
*edit: As noted in the comment below (thanks) you FIRST need to earn your managers trust (5 stars) before they will consider this.
7- Get used to ridiculous trades. In 3 of My Life games so far ive seen Jeter get traded off the Yankees. Understandable from a defensive point of view, but would never happen in real life. Many other big names get traded, deal with it.
8- The "mood" of your player always drove me nuts, but its part of the game. For pitchers i notice a HUGE difference between pink (very happy) and purple (sluggish). A pitcher will lose a lot movement and 5-8 mph off their speed. For batters its harder to hit home runs, you run a little slower, and may have a harder time making solid contact.
9- General tip for those wondering "what should i do in my free time?" I stock up on workout equipment at my house so when i am playing home games i workout, and then bring some books to read on the road so its easier to gain exp through that.
10- In order to get the best agent in the game you need to become a B level player. His name is Darryl and the better relationship (more stars) you have with him, and with any agent, the better contract you will get. Suggestion? Darryl loves golf, so try and get to be a C golfer by the time you work with him.
11- For pitchers, you can add pitches to your by purchasing pitching books (not the beg / int / adv books but actual pitch books) and then when spring training comes around the next year you can learn them!
I will keep adding to this as i think of stuff, hit me up with any comments, questions, or corrections!