How to raise monthly income?

You should install lots of public transportation and try not to give too much funds to places like schools,police stations etc. But don't give them too little or they'll go on strike

By lowering taxes for. All levels of residential you will see an increase in population growth and taxes made for income. Focus on mainting your budget inbetween 3k-4k until you hit 10k in population. Consistently check you rci chart and build accordingly......
Once you have managed to reach 10k in population then it is time to readjust taxes according to your number in each res. Level...

Low income from 7% then for more growth lower it to 6.5% and so forth. You will keep demand up bu doing this every 10k pop. Until you reach 80k-90k then you will really be focused on having a set rate of tax.

Good luck mayor

You should also provide parks, as it can raise the land value further, moving high wealth sims in. If you have multiple cities in the region, and one that neighbors is home to a lot of high-tech industry or huge offices, your people will commute to there, (Install NAM) to get more money, leading your cities average income to spike.