What are Sodapop's plans for himself and Sandy in The Outsiders?

The answer to this question can be found at the very end of chapter one. Ponyboy and Sodapop are going to sleep for the night, and they talk to each other as they are falling asleep. A lot of Soda's comments are about how Darry really does love Ponyboy, but Ponyboy doesn't really buy it. Ponyboy then changes the subject and asks Soda why he dropped out of school. Soda doesn't give a big convoluted answer. He simply says he dropped out because he thinks he dumb.

"Cause I'm dumb. The only things I was passing anyway were auto mechanics and gym."

Soda then changes the subject by telling Ponyboy his future plans about him and Sandy. He says that he loves her, and he is planning on marrying her. He then admits that he will have to wait until she gets out of high school and he gets a better job. Sodapop also thinks that he might wait long enough for Pony to get out of school too. That way Soda can help Darry with the bills until Pony is old enough to do that himself after high school.

"I think I'm gonna marry Sandy. After she gets out of school and I get a better job and everything. I might wait till you get out of school, though. So I can still help Darry with the bills and stuff."

At the end of Chapter 1, Sodapop is lying in bed next to Ponyboy. Ponyboy asks him why he dropped out of school and Sodapop tells Pony that he dropped out of school because he's dumb. He laughs it off and says that he plans to marry his girlfriend, Sandy, after she gets out of school and he gets a better job. Later on in the novel, we find out that Sandy left town to live with her grandmother in Florida. Steve says that her parents didn't approve of her marrying Sodapop, so they made her leave town. In Chapter 12, it is implied that Sandy had gotten pregnant, and that was the reason her parents made her move to Florida. In the 1960s, premarital relations were frowned upon and Sandy's parents probably wanted to keep her pregnancy a secret. Regardless, Sodapop is devastated that the girl he plans on marrying has moved to Florida.

What is implied but not stated about Sodapop's and Sandy's situation on page 111 of The Outsiders?

In the discussion about having a party after the rumble to celebrate stomping the Socs, Ponyboy innocently inquires if Sodapop would bring Sandy to the party.  Unbeknownst to Ponyboy, Sandy had to go and live with her grandmother in Florida.  When Ponyboy still does not understand the circumstances of her departure, Steve gets angry:

"Does he have to draw you a picture? It was eigher that or get married, and her parents almost hit the roof a the idea of her marryin' a sixteen-year-old kid" (111). 

Based on the evidence provided by Hinton, the reader can conclude that Sandy had to move to Florida because she was pregnant, and this scenario fits well with the historical context of the novel.  Teen girls who found themselves pregnant out of wedlock were often shipped to a relative's house out of state, usually with the idea that they could discreetly have the baby there, put the child up for adoption, and then move home with no one in the community being any wiser. 

Interestingly enough, Darry reveals later that Sodapop was not the father of Sandy's baby, but he still "wanted to marry her anyway, but she just left" (174).

What is implied about Sodapop's and Sandy's situation in The Outsiders?

I think that you are referring to what is said about Soda and Sandy towards the very end of the book.  Soda is said to be all depressed because Sandy has left him and left Tulsa and headed to Florida.  That much is stated clearly.

What is implied is that Sandy is pregnant and the baby is not Soda's.  I infer that because it says in the book

I guess she didn't love him like he thought she did, because it wasn't him.

I can't be sure, but I think this means that she is pregnant but is was not him that got her pregnant.

In my copy of the book, this is on page 175 out of 180 total pages -- you can look it up and see if you agree.