Whats the best way to start investing in farming in kenya

If you  want to invest in farming, you would most likely need to partner up with a Kenyan.  For one thing, foreigners aren't permitted to buy agricultural land.

An alternative is a type of farming that doesn't need much space such as Quails, mushrooms and so on. 

Check out the rules on work permits too.  I understand that the government are currently on a drive to review foreigners work permits over concerns that too many foreigners are in jobs that Kenyans could do.

Best advice is to do your research and do NOT rush into any commitments.
Land is an is a touchy issue in Kenya fraught with corrupt con men.
Many have lost their investments even after following proper rules and procedures only to be told after that their ownership papers are worthless.
Just Google this and you will see all the many cases in front of the courts.
So please protect your self and good luck!