Partner opens a suit

With no major suit fit to bid, responder's second priority is bidding 1NT if possible as the points for a NoTrump contract score are higher than a Minor suit contract

Bids holding a minimum hand (6-9- points)

Usually without support for your partner's suit, you want to see if a NoTrump contract is viable. Bidding a new suit at the 1-level shows your partner you have at least 6 HCP.
Keep the bidding as low as you can to leave more room to explore the correct place for your contract. Your choices are usually one of the following.

Bid (show) a suit at the 1-level if possible 6+ HCP
Bid 1NT to show you have 6 - 9 HCP

No Trump bids and suit raises are Limit bids - Limit bids are useful bids as they show the precise strength and the shape of a hand.

Bidding game directly is a preemptive bid designed to shut out the opponents it also asks your partner stop bidding.

Leaping into game promises 6-9 HCP with either 4 trumps & a void(zero cards in a suit), or 5 trumps and a singleton (one card in a suit).
If you have more than 13 HCP, you must not preempt.... you should change suit at the lowest level first and then bid game (this is known as a delayed game raise).

Responses with a minimum hand

Opening Bid	Your Response	Points	Meaning of Responder Bid
1 in a suit	Pass	0-6	Game is out of reach and you have no five card suit to bid
1 in a suit	1NT	6-9	Not able to bid at the two level and no four card suit higher ranked than openers suit
1H/1S	4H/4S	5-7	Promises five cards with minimum strength points
1H/1S/1D/1C	Intervening bid by opposition and you bid double "X"	5-7	A negative Double promises 8-9 points and some cards but unable to bid over the opposition's bid