Is there any API that i can send to it the rsID (e.g. rs4988235) and the API respond with the position (i.e. chromosome number and position of the nucleotide) and the effect related to this rsID (e.g. for Rs4988235, CC: likely to be lactose intolerant as an adult. CT: likely to be able to digest milk as an adult. TT: can digest milk.)
Yes, there is the Ensembl REST APIs. Two endpoints I could think of to get the data you are after are GET variation/:species/:ID (mainly) and the GET vep/:species/id/:id, which gives the added value of the annotation of your variant by the VEP. Check a previous biostars post.
The dbSNP database can be queried for location and chromosome by RS#. You can download the dbSNP annotation and run queries on it, or maybe find an online resource.
For phenotype, it's much harder to be certain. The ClinVar database is another you can download that has a lot of those annotations, but most SNPs will not have known function.