Unfiortunatelty most subway stations do not have escalators or elevators and require descending or climbing a couple of flight of steep, narrow stone stairs. If this is an issue you can check the mta web site to see if the stations you need do have them.
For getting to Citi Field you can also use the LIRR from Penn Station and you can use Metro North to Yankee Stadium. But do check the info on the stations to see if any steps or walking will be an issue. (You are not clear how much walking is a problem - is several blocks OK? Or do you need to limit it to very short distances - such as from the taxi to the stadium entrances.
For Manhattan the regular city buses can be helpful since they have only a couple of steps and can kneel if necessary - but they are much slower than the subway since they stop every couple of blocks.
If you have additional questions, you may want to post in the NY City forum to attract more readers; this is the general forum for all of NY State.
Thank you. A couple of blocks is OK. More concerned with many stairs to subway