First, let's take a look at the "UU Viability Thread," where you can find tiers for Pokemon and talk to people about how Pokemon perform in a tier, and where they should be placed. Higher tiered Pokemon are Pokemon that are more likely played more often, and generally better so that's a main area to look at when making a team. Right from the get-go, you should look at your team and know what their weaknesses are first. For example, your first four Pokemon I realized are incredibly weak to Slowking, where Kangaskhan can at best check Slowking, and everything else loses to it or at best can do a little bit of damage to it.
So now, you know one weakness of your team in general, so how do you take care of that threat? You can change your Pokemon movesets, or in your case, add another Pokemon to the team. You said you have a Raichu, but it may be a tad better to train an Exeggutor or a Manectric. Exeggutor all around can work well with your team, considering your lack of both Grass and Psychic, and can check Slowbro very well. On the off-side, Manectric gives you a better offensive option, with higher speed. The choice is up to you. You have your fifth Pokemon now, so do the same thing for your sixth.
So you have your team now, but instead of heading into battle, you should double-check your entire team. Ask yourself questions like "Do stall-teams beat me? Are there Pokemon that are common so I should considering changing "x?," etc.
Anyway, that's just teambuilding advice, since I'm not completely familiar with the current meta in this game (I only started like 2 weeks ago okay) and just giving you tips off my competitive experience in Showdown. I'm sure someone else can give you more advice on specifics like EVs and movesets. Regardless, good luck!